PHP: a fractal of bad design-jiangwen127-ChinaUnix博客

Preface

I’m cranky. I complain about a lot of things. There’s a lot in the world of technology I don’t like, and that’s really to be expected—programming is a hilariously young discipline, and none of us have the slightest clue what we’re doing. Combine with , and I have a lifetime’s worth of stuff to gripe about.

This is not the same. PHP is not merely awkward to use, or ill-suited for what I want, or suboptimal, or against my religion. I can tell you all manner of good things about languages I avoid, and all manner of bad things about languages I enjoy. Go on, ask! It makes for interesting conversation.

PHP is the lone exception. Virtually every feature in PHP is broken somehow. The language, the framework, the ecosystem, are all just bad. And I can’t even point out any single damning thing, because the damage is so systemic. Every time I try to compile a list of PHP gripes, I get stuck in this depth-first search discovering more and more appalling trivia. (Hence, fractal.)

PHP is an embarrassment, a blight upon my craft. It’s so broken, but so lauded by every empowered amateur who’s yet to learn anything else, as to be maddening. It has paltry few redeeming qualities and I would prefer to forget it exists at all.

But I’ve got to get this out of my system. So here goes, one last try.

An analogy

I just blurted this out to Mel to explain my frustration and she insisted that I reproduce it here.

I can’t even say what’s wrong with PHP, because— okay. Imagine you have uh, a toolbox. A set of tools. Looks okay, standard stuff in there.

You pull out a screwdriver, and you see it’s one of those weird tri-headed things. Okay, well, that’s not very useful to you, but you guess it comes in handy sometimes.

You pull out the hammer, but to your dismay, it has the claw part on both sides. Still serviceable though, I mean, you can hit nails with the middle of the head holding it sideways.

You pull out the pliers, but they don’t have those serrated surfaces; it’s flat and smooth. That’s less useful, but it still turns bolts well enough, so whatever.

And on you go. Everything in the box is kind of weird and quirky, but maybe not enough to make it completely worthless. And there’s no clear problem with the set as a whole; it still has all the tools.

Now imagine you meet millions of carpenters using this toolbox who tell you “well hey what’s the problem with these tools? They’re all I’ve ever used and they work fine!” And the carpenters show you the houses they’ve built, where every room is a pentagon and the roof is upside-down. And you knock on the front door and it just collapses inwards and they all yell at you for breaking their door.

That’s what’s wrong with PHP.

Stance

I assert that the following qualities are important for making a language productive and useful, and PHP violates them with wild abandon. If you can’t agree that these are crucial, well, I can’t imagine how we’ll ever agree on much.

A language must be predictable. It’s a medium for expressing human ideas and having a computer execute them, so it’s critical that a human’s understanding of a program actually be correct.
A language must be consistent. Similar things should look similar, different things different. Knowing part of the language should aid in learning and understanding the rest.
A language must be concise. New languages exist to reduce the boilerplate inherent in old languages. (We could all write machine code.) A language must thus strive to avoid introducing new boilerplate of its own.
A language must be reliable. Languages are tools for solving problems; they should minimize any new problems they introduce. Any “gotchas” are massive distractions.
A language must be debuggable. When something goes wrong, the programmer has to fix it, and we need all the help we can get.

My position is thus:

PHP is full of surprises: mysql_real_escape_string, E_ACTUALLY_ALL
PHP is inconsistent: strpos, str_rot13
PHP requires boilerplate: error-checking around C API calls, ===
PHP is flaky: ==, for ($foo as &$bar)
PHP is opaque: no stack traces, complex error reporting

I can’t provide a paragraph of commentary for every single issue explaining why it falls into these categories, or this would be endless. I trust the reader to, like, think.

Don’t comment with these things

I’ve been in PHP arguments a lot. I hear a lot of very generic counter-arguments that are really only designed to halt the conversation immediately. Don’t pull these on me, please. :(

Do not tell me that “good developers can write good code in any language”, or bad developers blah blah. That doesn’t mean anything. A good carpenter can drive in a nail with either a rock or a hammer, but how many carpenters do you see bashing stuff with rocks? Part of what makes a good developer is the ability to choose the tools that work best.
Do not tell me that it’s the developer’s responsibility to memorize a thousand strange exceptions and surprising behaviors. Yes, this is necessary in any system, because computers suck. That doesn’t mean there’s no upper limit for how much zaniness is acceptable in a system. PHP is nothing but exceptions, and it is not okay when wrestling the language takes more effort than actually writing your program. My tools should not create net positive work for me to do.
Do not tell me “that’s how the C API works”. What on Earth is the point of using a high-level language if all it provides are some string helpers and a ton of verbatim C wrappers? Just write C! Here, there’s even a for it.
Do not tell me “that’s what you get for doing weird things”. If two features exist, someday, someone will find a reason to use them together. And again, this isn’t C; there’s no spec, there’s no need for “undefined behavior”.
Do not tell me that Facebook and Wikipedia are built in PHP. I’m aware! They could also be written in Brainfuck, but as long as there are smart enough people wrangling the things, they can overcome problems with the platform. For all we know, development time could be halved or doubled if these products were written in some other language; this data point alone means nothing.
Ideally, don’t tell me anything! This is my one big shot; if this list doesn’t hurt your opinion of PHP, nothing ever will, so stop arguing with some dude on the Internet and go make a cool website in record time to prove me wrong :)

Side observation: I loooove Python. I will also happily talk your ear off complaining about it, if you really want me to. I don’t claim it’s perfect; I’ve just weighed its benefits against its problems and concluded it’s the best fit for things I want to do.

And I have never met a PHP developer who can do the same with PHP. But I’ve bumped into plenty who are quick to apologize for anything and everything PHP does. That mindset is terrifying.

PHP Core language

CPAN has been called the “standard library of Perl”. That doesn’t say much about Perl’s standard library, but it makes the point that a solid core can build great things.

Philosophy

PHP was originally designed explicitly for non-programmers (and, reading between the lines, non-programs); it has not well escaped its roots. A choice quote from the PHP 2.0 documentation, regarding + and friends doing type conversion:

Once you start having separate operators for each type you start making the language much more complex. ie. you can’t use ‘==’ for stings [sic], you now would use ‘eq’. I don’t see the point, especially for something like PHP where most of the scripts will be rather simple and in most cases written by non-programmers who want a language with a basic logical syntax that doesn’t have too high a learning curve.
PHP is built to keep chugging along at all costs. When faced with either doing something nonsensical or aborting with an error, it will do something nonsensical. Anything is better than nothing.
There’s no clear design philosophy. Early PHP was inspired by Perl; the huge stdlib with “out” params is from C; the OO parts are designed like C++ and Java.
PHP takes vast amounts of inspiration from other languages, yet still manages to be incomprehensible to anyone who knows those languages. (int) looks like C, but int doesn’t exist. Namespaces use \. The new array syntax results in [key => value], unique among every language with hash literals.
Weak typing (i.e., silent automatic conversion between strings/numbers/et al) is so complex that whatever minor programmer effort is saved is by no means worth it.
Little new functionality is implemented as new syntax; most of it is done with functions or things that look like functions. Except for class support, which deserved a slew of new operators and keywords.
Some of the problems listed on this page do have first-party solutions—if you’re willing to pay Zend for fixes to their open-source programming language.
There is a whole lot of action at a distance. Consider this code, taken from the PHP docs somewhere.
@fopen('', 'r');
What will it do?
- If PHP was compiled with --disable-url-fopen-wrapper, it won’t work. (Docs don’t say what “won’t work” means; returns null, throws exception?) Note that this flag was removed in PHP 5.2.5.
- If allow_url_fopen is disabled in php.ini, this still won’t work. (How? No idea.)
- Because of the @, the warning about the non-existent file won’t be printed.
- But it will be printed if scream.enabled is set in php.ini.
- Or if scream.enabled is set manually with ini_set.
- But not if the right error_reporting level isn’t set.
- If it is printed, exactly where it goes depends on display_errors, again in php.ini. Or ini_set.
I can’t tell how this innocuous function call will behave without consulting compile-time flags, server-wide configuration, and configuration done in my program. And this is all built in behavior.
The language is full of global and implicit state. mbstring uses a global character set. func_get_arg and friends look like regular functions, but operate on the currently-executing function. Error/exception handling have global defaults. register_tick_function sets a global function to run every tick—what?!
There is no threading support whatsoever. (Not surprising, given the above.) Combined with the lack of built-in fork (mentioned below), this makes parallel programming extremely difficult.
Parts of PHP are practically designed to produce buggy code.
- json_decode returns null for invalid input, even though null is also a perfectly valid object for JSON to decode to—this function is completely unreliable unless you also call json_last_error every time you use it.
- array_search, strpos, and similar functions return 0 if they find the needle at position zero, but false if they don’t find it at all.
Let me expand on that last part a bit.

In C, functions like strpos return -1 if the item isn’t found. If you don’t check for that case and try to use that as an index, you’ll hit junk memory and your program will blow up. (Probably. It’s C. Who the fuck knows. I’m sure there are tools for this, at least.)

In, say, Python, the equivalent .index methods will raise an exception if the item isn’t found. If you don’t check for that case, your program will blow up.

In PHP, these functions return false. If you use FALSE as an index, or do much of anything with it except compare with ===, PHP will silently convert it to 0 for you. Your program will not blow up; it will, instead, do the wrong thing with no warning, unless you remember to include the right boilerplate around every place you use strpos and certain other functions.

This is bad! Programming languages are tools; they’re supposed to work with me. Here, PHP has actively created a subtle trap for me to fall into, and I have to be vigilant even with such mundane things as string operations and equality comparison. PHP is a minefield.

I have heard a great many stories about the PHP interpreter and its developers from a great many places. These are from people who have worked on the , debugged PHP core, interacted with core developers. Not a single tale has been a compliment.

So I have to fit this in here, because it bears repeating: PHP is a community of amateurs. Very few people designing it, working on it, or writing code in it seem to know what they’re doing. (Oh, dear reader, you are of course a rare exception!) Those who do grow a clue tend to drift away to other platforms, reducing the average competence of the whole. This, right here, is the biggest problem with PHP: it is absolutely the blind leading the blind.

Okay, back to facts.

Operators

== is .
- It’s not transitive. "foo" == TRUE, and "foo" == 0… but, of course, TRUE != 0.
- == converts to numbers when possible, which means it converts to floats when possible. So large hex strings (like, say, password hashes) may occasionally .
- For the same reason, "6" == " 6", "4.2" == "4.20", and "133" == "0133". But note that 133 != 0133, because 0133 is octal.
- === compares values and type… except with objects, where === is only true if both operands are actually the same object! For objects, == compares both value (of every attribute) and type, which is what === does for every other type. What.
The comparison operators try to sort arrays, two different ways: first by length, then by elements. If they have the same number of elements but different sets of keys, though, they are uncomparable.
Objects compare as greater than anything else… except other objects, which they are neither less than nor greater than.
For a more type-safe ==, we have ===. For a more type-safe <, we have… nothing. "123" < "0124", always, no matter what you do.
Despite the craziness above, and the explicit rejection of Perl’s pairs of string and numeric operators, PHP does not overload +. + is always addition, and . is always concatenation.
The [] indexing operator can also be spelled {}.
[] can be used on any variable, not just strings and arrays. It returns null and issues no warning.
[] cannot slice; it only retrieves individual elements.
foo()[0] is a syntax error. (Fixed in PHP 5.4.)
Unlike (literally!) every other language with a similar operator, ?: is left associative. So this:
$arg = 'T'; $vehicle = ( ( $arg == 'B' ) ? 'bus' : ( $arg == 'A' ) ? 'airplane' : ( $arg == 'T' ) ? 'train' : ( $arg == 'C' ) ? 'car' : ( $arg == 'H' ) ? 'horse' : 'feet' ); echo $vehicle;
prints horse.

Variables

There is no way to declare a variable. Variables that don’t exist are created with a null value when first used.
Global variables need a global declaration before they can be used. This is a natural consequence of the above, so it would be perfectly reasonable, except that globals can’t even be read without an explicit declaration—PHP will quietly create a local with the same name, instead. I’m not aware of another language with similar scoping issues.
There are no references. What PHP calls references are really aliases; there’s nothing that’s a step back, like Perl’s references, and there’s no pass-by-object identity like in Python.
“Referenceness” infects a variable unlike anything else in the language. PHP is dynamically-typed, so variables generally have no type… except references, which adorn function definitions, variable syntax, and assignment. Once a variable is made a reference (which can happen anywhere), it’s stuck as a reference. There’s no obvious way to detect this and un-referencing requires nuking the variable entirely.
Okay, I lied. There are ”” which also infect variables: $x = new SplBool(true); $x = "foo"; will fail. This is like static typing, you see.
A reference can be taken to a key that doesn’t exist within an undefined variable (which becomes an array). Using a non-existent array normally issues a notice, but this does not.
Constants are defined by a function call taking a string; before that, they don’t exist. (This may actually be a copy of Perl’s use constant behavior.)
Variable names are case-sensitive. Function and class names are not. This includes method names, which makes camelCase a strange choice for naming.

Constructs

array() and a few dozen similar constructs are not functions. array on its own means nothing, $func = "array"; $func(); doesn’t work.
Array unpacking can be done with the list($a, $b) = ... operation. list() is function-like syntax just like array. I don’t know why this wasn’t given real dedicated syntax, or why the name is so obviously confusing.
(int) is obviously designed to look like C, but it’s a single token; there’s nothing called int in the language. Try it: not only does var_dump(int) not work, it throws a parse error because the argument looks like the cast operator.
(integer) is a synonym for (int). There’s also (bool)/(boolean) and (float)/(double)/(real).
There’s an (array) operator for casting to array. Given that PHP has no other structure type, I don’t know why this exists.
include() and friends are basically C’s #include: they dump another source file into yours. There is no module system, even for PHP code.
There’s no such thing as a nested or locally-scoped function or class. They’re only global. Including a file dumps its variables into the current function’s scope (and gives the file access to your variables), but dumps functions and classes into global scope.
Appending to an array is done with $foo[] = $bar.
echo is a statement-y kind of thing, not a function.
empty($var) is so extremely not-a-function that anything but a variable, e.g. empty($var || $var2), is a parse error. Why on Earth does the parser need to know about empty?
There’s redundant syntax for blocks: if (...): ... endif;, etc.

Error handling

PHP’s one unique operator is @ (actually borrowed from DOS), which silences errors.
PHP errors don’t provide stack traces.
PHP parse errors generally just spew the parse state and nothing more, making a forgotten quote .
PHP’s parser refers to e.g. :: internally as T_PAAMAYIM_NEKUDOTAYIM, and the << operator as T_SL. I say “internally”, but as above, this is what’s shown to the programmer when :: or << appears in the wrong place.
Most error handling is in the form of printing a line to a server log nobody reads and carrying on.
E_STRICT is a thing, but it doesn’t seem to actually prevent much and there’s no documentation on what it actually does.
E_ALL includes all error categories—except E_STRICT.
Weirdly inconsistent about what’s allowed and what isn’t. I don’t know how E_STRICT applies here, but these things are okay:
- Trying to access a non-existent object property, i.e., $foo->x. (warning)
- Using a variable as a function name, or variable name, or class name. (silent)
- Trying to use an undefined constant. (notice)
- Trying to access a property of something that isn’t an object. (notice)
- Trying to use a variable name that doesn’t exist. (notice)
- 2 < "foo" (silent)
- foreach (2 as $foo); (warning)
And these things are not:
- Trying to access a non-existent class constant, i.e., $foo::x. (fatal error)
- Using a constant string as a function name, or variable name, or class name. (parse error)
- Trying to call an undefined function. (fatal error)
- Leaving off a semicolon on the last statement in a block or file. (parse error)
- Using list and various other quasi-builtins as method names. (parse error)
- Subscripting the return value of a function, i.e., foo()[0]. (parse error; okay in 5.4, see above)
There are a good few examples of other weird parse errors elsewhere in this list.
The __toString method can’t throw exceptions. If you try, PHP will… er, throw an exception. (Actually a fatal error, which would be passable, except…)
PHP errors and PHP exceptions are completely different beasts. They don’t seem to interact at all.
- PHP errors (internal ones, and calls to trigger_error) cannot be caught with try/catch.
- Likewise, exceptions do not trigger error handlers installed by set_error_handler.
- Instead, there’s a separate set_exception_handler which handles uncaught exceptions, because wrapping your program’s entry point in a try block is impossible in the mod_php model.
- Fatal errors (e.g., new ClassDoesntExist()) can’t be caught by anything. A lot of fairly innocuous things throw fatal errors, forcibly ending your program for questionable reasons. Shutdown functions still run, but they can’t get a stack trace (they run at top-level), and they can’t easily tell if the program exited due to an error or running to completion.
There is no finally construct, making wrapper code (set handler, run code, unset handler; monkeypatch, run a test, unmonkeypatch) tedious and difficult to write. Despite that OO and exceptions were largely copied from Java, this , because finally “doesn’t make much sense in the context of PHP”. Huh?

Functions

Function calls are apparently rather .
Some built-in functions interact with reference-returning functions in, er, .
As mentioned elsewhere, a lot of things that look like functions or look like they should be functions are actually language constructs, so nothing that works with functions will work with them.
Function arguments can have “type hints”, which are basically just static typing. But you can’t require that an argument be an int or string or object or other “core” type, even though every builtin function uses this kind of typing, probably because int is not a thing in PHP. (See above about (int).) You also can’t use the special used heavily by builtin functions: mixed, number, or callback.
- As a result, this:
  function foo(string $s) {} foo("hello world");
  produces the error:
  PHP Catchable fatal error: Argument 1 passed to foo() must be an instance of string, string given, called in...
- You may notice that the “type hint” given doesn’t actually have to exist; there is no string class in this program. If you try to use ReflectionParameter::getClass() to examine the type hint dynamically, then it will balk that the class doesn’t exist, making it impossible to actually retrieve the class name.
- A function’s return value can’t be hinted.
Passing the current function’s arguments to another function (dispatch, not uncommon) is done by call_user_func_array('other_function', func_get_args()). But func_get_args throws a fatal error at runtime, complaining that it can’t be a function parameter. How and why is this even a type of error? (Fixed in PHP 5.3.)
Closures require explicitly naming every variable to be closed-over. Why can’t the interpreter figure this out? Kind of hamstrings the whole feature.
No named arguments to functions. Actually by the devs because it “makes for messier code”.
Function arguments with defaults can appear before function arguments without, even though the documentation points out that this is both weird and useless. (So why allow it?)
Extra arguments to a function are ignored (except with builtin functions, which raise an error). Missing arguments are assumed null.
“Variadic” functions require faffing about with func_num_args, func_get_arg, and func_get_args. There’s no syntax for such a thing.

The functional parts of PHP are designed like C, but the objectional (ho ho) parts are designed like Java. I cannot overemphasize how jarring this is. I’ve yet to find a global function that even has a capital letter in its name, yet use camelCase method names and have getFoo Java-style accessors. This is a dynamic language, right? Perl, Python, and Ruby all have some concept of “property” access via code; PHP has only the clunky __get and friends. The class system is designed around the lower-level Java language which is naturally and deliberately more limited than PHP’s contemporaries, and I am baffled.
Classes are not objects. Any metaprogramming has to refer to them by string name, just like functions.
Built-in types are not objects and (unlike Perl) can in no way be made to look like objects.
instanceof is an operator, despite that classes were a late addition and most of the language is built on functions and function-ish syntax. Java influence? Classes not first-class? (I don’t know if they are.)
- But there is an is_a function. With an optional argument specifying whether to allow the object to actually be a string naming a class.
- get_class is a function; there’s no typeof operator. Likewise is_subclass_of.
- This doesn’t work on builtin types, though (again, int is not a thing). For that, you need is_int etc.
- Also the right-hand side has to be a variable or literal string; it can’t be an expression. That causes… a parse error.
clone is an operator?!
OO design is a weird mix of Java and Perl.
Object attributes are $obj->foo, but class attributes are $obj::foo. I’m not aware of another language that does this or how it’s useful.
Also, an instance method can still be called statically (Class::method()). If done so from another method, this is treated like a regular method call on the current $this. I think.
new, private, public, protected, static, etc. Trying to win over Java developers? I’m aware this is more personal taste, but I don’t know why this stuff is necessary in a dynamic language—in C++ most of it’s about compilation and compile-time name resolution.
Subclasses cannot override private methods. Subclass overrides of public methods can’t even see, let alone call, the superclass’s private methods. Problematic for, say, test mocks.
Methods cannot be named e.g. “list”, because list() is special syntax (not a function) and the parser gets confused. There’s no reason this should be ambiguous, and monkeypatching the class works fine. ($foo->list() is not a syntax error.)
If an exception is thrown while evaluating a constructor’s arguments (e.g., new Foo(bar()) and bar() throws), the constructor won’t be called, but the destructor will be. (This is fixed in PHP 5.3.)
Exceptions in __autoload and destructors cause fatal errors.
There are no constructors or destructors. __construct is an initializer, like Python’s __init__. There is no method you can call on a class to allocate memory and create an object.
There is no default initializer. Calling parent::__construct() if the superclass doesn’t define its own __construct is a fatal error.
OO brings with it an iterator interface that parts of the language (e.g., for...as) respect, but nothing built-in (like arrays) actually implements the interface. If you want an array iterator, you have to wrap it in an ArrayIterator. There are no built-in ways to chain or slice or otherwise work with iterators as first-class objects.
Classes can overload how they convert to strings and how they act when called, but not how they convert to numbers or any other builtin type.
Strings, numbers, and arrays all have a string conversion; the language relies heavily on this. Functions and classes are strings. Yet trying to convert a built-in or user-defined object (even a Closure) to a string causes an error if it doesn’t define __toString. Even echo becomes potentially error-prone.
There is no overloading for equality or ordering.
Static variables inside instance methods are global; they share the same value across all instances of the class.

Standard library

Perl is “some assembly required”. Python is “batteries included”. PHP is “kitchen sink, but it’s from Canada and ”.

General

There is no module system. You can compile PHP extensions, but which ones are loaded is specified by php.ini, and your options are for an extension to exist (and inject its contents into your global namespace) or not.
As namespaces are a recent feature, the standard library isn’t broken up at all. There are thousands of functions in the global namespace.
Chunks of the library are wildly inconsistent from one another.
- Underscore versus not: strpos/str_rot13, php_uname/phpversion, base64_encode/urlencode, gettype/get_class
- “to” versus 2: ascii2ebcdic, bin2hex, deg2rad, strtolower, strtotime
- Object+verb versus verb+object: base64_decode, str_shuffle, var_dump versus create_function, recode_string
- Argument order: array_filter($input, $callback) versus array_map($callback, $input), strpos($haystack, $needle) versus array_search($needle, $haystack)
- Prefix confusion: usleep versus microtime
- Case insensitive functions vary on where the i goes in the name.
- About half the array functions actually start with array_. The others do not.
Kitchen sink. The libary includes:
- Bindings to ImageMagick, bindings to GraphicsMagick (which is a fork of ImageMagick), and a handful of functions for inspecting EXIF data (which ImageMagick can already do).
- Functions for parsing bbcode, a very specific kind of markup used by a handful of particular forum packages.
- Way too many XML packages. DOM (OO), DOM XML (not), libxml, SimpleXML, “XML Parser”, XMLReader/XMLWriter, and half a dozen more acronyms I can’t identify. There’s surely some kind of difference between these things and you are free to go figure out what that is.
- Bindings for two particular credit card processors, SPPLUS and MCVE. What?
- Three ways to access a MySQL database: mysql, mysqli, and the PDO abstraction thing.

C influence

This deserves its own bullet point, because it’s so absurd yet permeates the language. PHP is a high-level, dynamically-typed programming language. Yet a massive portion of the standard library is still very thin wrappers around C APIs, with the following results:

“Out” parameters, even though PHP can return ad-hoc hashes or multiple arguments with little effort.
At least a dozen functions for getting the last error from a particular subsystem (see below), even though PHP has had exceptions for eight years.
Warts like mysql_real_escape_string, even though it has the same arguments as the broken mysql_escape_string, just because it’s part of the MySQL C API.
Global behavior for non-global functionality (like MySQL). Using multiple MySQL connections apparently requires passing a connection handle on every function call.
The wrappers are really, really, really thin. For example, calling dba_nextkey without calling dba_firstkey will segfault.
There’s a set of ctype_* functions (e.g. ctype_alnum) that map to the C character-class detection functions of similar names. Most of these functions have equivalents outside this group, and it’s not clear why the ctype versions are different, nor why the name is confusingly similar to the existing well-known “ctypes”.

Genericism

There is none. If a function might need to do two slightly different things, PHP just has two functions.

How do you sort backwards? In Perl, you might do sort { $b <=> $a }. In Python, you might do .sort(reverse=True). In PHP, there’s a separate function called rsort().

Functions that look up a C error: curl_error, json_last_error, openssl_error_string, imap_errors, mysql_error, xml_get_error_code, bzerror, date_get_last_errors, others?
Functions that sort: array_multisort, arsort, asort, ksort, krsort, natsort, natcasesort, sort, rsort, uasort, uksort, usort
Functions that find text: ereg, eregi, mb_ereg, mb_eregi, preg_match, strstr, strchr, stristr, strrchr, strpos, stripos, strrpos, strripos, mb_strpos, mb_strrpos, plus the variations that do replacements
There are a lot of aliases as well, which certainly doesn’t help matters: strstr/strchr, is_int/is_integer/is_long, is_float/is_double, pos/current, sizeof/count, chop/rtrim, implode/join, die/exit, trigger_error/user_error…
scandir returns a list of files within a given directory. Rather than (potentially usefully) return them in directory order, the function returns the files already sorted. And there’s an optional argument to get them in reverse alphabetical order. There were not, apparently, enough sort functions.
str_split breaks a string into chunks of equal length. chunk_split breaks a string into chunks of equal length, then joins them together with a delimiter.
Reading archives requires a separate set of functions depending on the format. There are six separate groups of such functions, all with different APIs, for bzip2, LZF, phar, rar, zip, and gzip/zlib.
Because calling a function with an array as its arguments is so awkward (call_user_func_array), there are some pairings like printf/vprintf and sprintf/vsprintf. These do the same things, but one function takes arguments and the other takes an array of arguments.

Text

preg_replace with the /e (eval) flag will do a string replace of the matches into the replacement string, then eval it.
strtok is apparently designed after the equivalent C function, which is already a bad idea for various reasons. Nevermind that PHP can easily return an array (whereas this is awkward in C), or that the very hack strtok(3) uses (modifying the string in-place) isn’t used here.
parse_str parses a query string, with no indication of this in the name. Also it acts just like register_globals and dumps the query into your local scope as variables, unless you pass it an array to populate. (It returns nothing, of course.)
explode refuses to split with an empty delimiter. Every other string split implementation anywhere takes this to mean the string should be split into characters; PHP instead has a totally separate function, confusingly called str_split and described as “converting a string to an array”.
For formatting dates, there’s strftime, which acts like the C API and respects locale. There’s also date, which has a completely different syntax and only works with English.
“ — Get line from gz-file pointer and strip HTML tags.” I’m dying to know the series of circumstances that led to this function’s conception.
mbstring
- It’s all about “multi-byte”, when the problem is character sets.
- Still operates on regular strings. Has a single global “default” character set. Some functions allow specifying charset, but then it applies to all arguments and the return value.
- Provides ereg_* functions, but those are deprecated. preg_* are out of luck, though they can understand UTF-8 by feeding them some PCRE-specific flag.

System and reflection

There are, in general, a whole lot of functions that blur the line between text and variables. compact and extract are just the tip of the iceberg.
There are several ways to actually be dynamic in PHP, and at a glance there are no obvious differences or relative benefits. classkit can modify user-defined classes; runkit supersedes it and can modify user-defined anything; the Reflection* classes can reflect on most parts of the language; there are a great many individual functions for reporting properties of functions and classes. Are these subsystems independent, related, redundant?
get_class($obj) returns the object’s class name. get_class() returns the name of the class the function is being called in. Setting aside that this one function does two radically different things: get_class(null)… acts like the latter. So you can’t trust it on an arbitrary value. Surprise!
The stream_* classes allow for implementing custom stream objects for use with fopen and other fileish builtins. “tell” cannot be implemented for . (Also there are of functions involved with this system.)
register_tick_function will accept a closure object. unregister_tick_function will not; instead it throws an error complaining that the closure couldn’t be converted to a string.
php_uname tells you about the current OS. Unless PHP can’t tell what it’s running on; then it tells you about the OS it was built on. It doesn’t tell you if this has happened.
fork and exec are not built in. They come with the pcntl extension, but that isn’t included by default. popen doesn’t provide a pid.
session_decode is for reading an arbitrary PHP session string, but it only works if there’s an active session already. And it dumps the result into $_SESSION, rather than returning it.

Miscellany

curl_multi_exec doesn’t change curl_errno on error, but it does change curl_error.
mktime’s arguments are, in order: hour, minute, second, month, day, year.

Data manipulation

Programs are nothing more than big machines that chew up data and spit out more data. A great many languages are designed around the kinds of data they manipulate, from awk to Prolog to C. If a language can’t handle data, it can’t do anything.

Numbers

Integers are signed and 32-bit on 32-bit platforms. Unlike all of PHP’s contemporaries, there is no automatic bigint promotion. So your math might work differently based on CPU architecture. Your only option for larger integers is to use the GMP or BC wrapper functions. (The developers have proposed . This is crazy.)
PHP supports octal syntax with a leading 0, so e.g. 012 will be the number ten. However, 08 becomes the number zero. The 8 (or 9) and any following digits disappear. 01c is a syntax error.
pi is a function. Or there’s a constant, M_PI.
There is , only the pow function.

Text

No Unicode support. Only ASCII will work reliably, really. There’s the mbstring extension, mentioned above, but it kinda blows.
Which means that using the builtin string functions on UTF-8 text risks corrupting it.
Similarly, there’s no concept of e.g. case comparisons outside of ASCII. Despite the proliferation of case-insensitive versions of functions, not one of them will consider é equal to É.
You can’t quote keys in variable interpolation, i.e., "$foo['key']" is a syntax error. You can unquote it (which would generate a warning anywhere else!), or use ${...}/{$...}.
"${foo[0]}" is okay. "${foo[0][0]}" is a syntax error. Putting the $ on the inside is fine with both. Bad copy of similar Perl syntax (with radically different semantics)?

Arrays

Oh, man.

This one datatype acts as a list, ordered hash, ordered set, sparse list, and occasionally some strange combination of those. How does it perform? What kind of memory use will there be? Who knows? Not like I have other options, anyway.
=> isn’t an operator. It’s a special construct that only exists inside array(...) and the foreach construct.
Negative indexing doesn’t work, since -1 is just as valid a key as 0.
Despite that this is the language’s only data structure, there is no shortcut syntax for it; array(...) is shortcut syntax. (PHP 5.4 is bringing “literals”, [...].)
The => construct is based on Perl, which allows foo => 1 without quoting. (That is, in fact, why it exists in Perl; otherwise it’s just a comma.) In PHP, you can’t do this without getting a warning; it’s the only language in its niche that has no vetted way to create a hash without quoting string keys.
Array functions often have confusing or inconsistent behavior because they have to operate on lists, hashes, or maybe a combination of the two. Consider array_diff, which “computers the difference of arrays”.
$first = array("foo" => 123, "bar" => 456); $second = array("foo" => 456, "bar" => 123); echo var_dump(array_diff($first, $second));
What will this code do? If array_diff treats its arguments as hashes, then obviously these are different; the same keys have different values. If it treats them as lists, then they’re still different; the values are in the wrong order.

In fact array_diff considers these equal, because it treats them like sets: it compares only values, and ignores order.
In a similar vein, array_rand has the strange behavior of selecting random keys, which is not that helpful for the most common case of needing to pick from a list of choices.
Despite how heavily PHP code relies on preserving key order:
array("foo", "bar") != array("bar", "foo")); array("foo" => 1, "bar" => 2) == array("bar" => 2, "foo" => 1));
I leave it to the reader to figure out what happens if the arrays are mixed. (I don’t know.)
array_fill cannot create zero-length arrays; instead it will issue a warning and return false.
All of the (many…) sort functions operate in-place and return nothing. There is no way to create a new sorted copy; you have to copy the array yourself, then sort it, then use the array.
But array_reverse returns a new array.
A list of ordered things and some mapping of keys to values sounds kind of like a great way to handle function arguments, but no.

Not arrays

The standard library includes “Quickhash”, an OO implementation of “specific strongly-typed classes” for implementing hashes. And, indeed, there are four classes, each dealing with a different combination of key and value types. It’s unclear why the builtin array implementation can’t optimize for these extremely common cases, or what the relative performance is.
There’s an ArrayObject class (which implements five different interfaces) that can wrap an array and have it act like an object. User classes can implement the same interfaces. But it only has a handful of methods, half of which don’t resemble built-in array functions, and built-in array functions don’t know how to operate on an ArrayObject or other array-like class.

Functions

Functions are not data. Closures are actually objects, but regular functions are not. You can’t even refer to them with their bare names; var_dump(strstr) issues a warning and assumes you mean the literal string, "strstr". There is no way to discern between an arbitrary string and a function “reference”.
create_function is basically a wrapper around eval. It creates a function with a regular name and installs it globally (so it will never be garbage collected—don’t use in a loop!). It doesn’t actually know anything about the current scope, so it’s not a closure. The name contains a NUL byte so it can never conflict with a regular function (because PHP’s parser fails if there’s a NUL in a file anywhere).
Declaring a function named __lambda_func will break create_function—the actual implementation is to eval-create the function named __lambda_func, then internally rename it to the broken name. If __lambda_func already exists, the first part will throw a fatal error.

Other

Incrementing (++) a NULL produces 1. Decrementing (--) a NULL produces NULL. Decrementing a string likewise leaves it unchanged.
There are no generators.

Web framework Execution

A single shared file, php.ini, controls massive parts of PHP’s functionality and introduces complex rules regarding what overrides what and when. PHP software that expects to be deployed on arbitrary machines has to override settings anyway to normalize its environment, which largely defeats the use of a mechanism like php.ini anyway.
PHP basically runs as CGI. Every time a page is hit, PHP recompiles the whole thing before executing it. Even dev servers for Python toy frameworks don’t act like this.

This has led to a whole market of “PHP accelerators” that just compile once, accelerating PHP all the way to any other language. Zend, the company behind PHP, has made this part of their .
For quite a long time, PHP errors went to the client by default—I guess to help during development. I don’t think this is true any more, but I still see the occasional mysql error spew at the top of a page.
PHP is full of strange “easter eggs” like . Not only is this completely irrelevant to building your application, but it allows detecting whether you’re using PHP (and perhaps roughly guessing what version), regardless of how much mod_rewrite, FastCGI, reverse proxying, or Server: configuration you’re doing.
Whitespace outside the tags, even in libraries, counts as literal text and is interpolated into the response (or causes “headers already sent” errors). A popular fix is to leave off the ?> closing token; PHP won’t complain, and there won’t be a trailing literal newline.

Deployment

Deployment is often cited as the biggest advantage of PHP: drop some files and you’re done. Indeed, that’s much easier than running a whole process as you may have to do with Python or Ruby or Perl. But PHP leaves plenty to be desired.

Across the board, I’m in favor of running Web applications as app servers and reverse-proxying to them. It takes minimal effort to set this up, and the benefits are plenty: you can manage your web server and app separately, you can run as many or few app processes on as many machines as you want without needing more web servers, you can run the app as a different user with zero effort, you can switch web servers, you can take down the app without touching the web server, you can do seamless deployment by just switching where a fifo points, etc. Welding your application to your web server is absurd and there’s no good reason to do it any more.

PHP is naturally tied to Apache. Running it separately, or with any other webserver, requires just as much mucking around (possibly more) as deploying any other language.
php.ini applies to every PHP application run anywhere. There is only one php.ini file, and it applies globally; if you’re on a shared server and need to change it, or if you run two applications that need different settings, you’re out of luck; you have to apply the union of all necessary settings and pare them down from inside the apps themselves using ini_set or in Apache’s configuration file or in .htaccess. If you can. Also wow that is a lot of places you need to check to figure out how a setting is getting its value.
Similarly, there is no easy way to “insulate” a PHP application and its dependencies from the rest of a system. Running two applications that require different versions of a library, or even PHP itself? Start by building a second copy of Apache.
The “bunch of files” approach, besides making routing a huge pain in the ass, also means you have to carefully whitelist or blacklist what stuff is actually available, because your URL hierarchy is also your entire code tree. Configuration files and other “partials” need C-like guards to prevent them from being loaded directly. Version control noise (e.g., .svn) needs protecting. With mod_php, everything on your filesystem is a potential entry point; with an app server, there’s only one entry point, and only the URL controls whether it’s invoked.
You can’t seamlessly upgrade a bunch of files that run CGI-style, unless you want crashes and undefined behavior as users hit your site halfway through the upgrade.
Despite how “simple” it is to configure Apache to run PHP, there are some subtle traps even there. While the PHP docs suggest using SetHandler to make .php files run as PHP, AddHandler appears to work just as well, and in fact Google gives me twice as many results for it. Here’s the problem.

When you use AddHandler, you are telling Apache that “execute this as php” is one possible way to handle .php files. But! Apache doesn’t have the same idea of file extensions that every human being on the planet does. It’s designed to support, say, index.html.en being recognized as both English and HTML. To Apache, a file can have any number of file extensions simultaneously.

Imagine you have a file upload form that dumps files into some public directory. To make sure nobody uploads PHP files, you just check that they don’t have a .php extension. All an attacker has to do is upload a file named foo.php.txt; your uploader won’t see a problem, but Apache will recognize it as PHP, and it will happily execute.

The problem here isn’t “using the original filename” or “not validating better”; the problem is that your web server is configured to run any old code it runs across—precisely the same property that makes PHP “easy to deploy”. CGI required +x, which was something, but PHP doesn’t even do that. And this is no theoretical problem; I’ve found multiple live sites with this issue.

Missing features

I consider all of these to be varying levels of critical for building a Web application. It seems reasonable that PHP, with its major selling point being that it’s a “Web language”, ought to have some of them.

No template system. There’s PHP itself, but nothing that acts as a big interpolator rather than a program.
No XSS filter. No, “remember to use htmlspecialchars” is not an XSS filter.
No CSRF protection. You get to do it yourself.
No generic standard database API. Stuff like PDO has to wrap every individual database’s API to abstract the differences away.
No routing. Your website looks exactly like your filesystem. Many developers have been tricked into thinking mod_rewrite (and .htaccess in general) is an acceptable substitute.
No authentication or authorization.
No dev server.
No interactive debugging.
No coherent deployment mechanism; only “copy all these files to the server”.

Security Language boundaries

PHP’s poor security reputation is largely because it will take arbitrary data from one language and dump it into another. This is a bad idea. "