php里include，include_once和requie，require_once的区别

这里是php。
1：带和不带once的区别：with the only difference being that if the code from a file has already been included, it will not be included again；*_once() should be used in cases where the same file might be included and evaluated more than once during a particular execution of a script, and you want to be sure that it is included exactly once to avoid problems with function redefinitions, variable value reassignments, etc.
2：require和include的区别：different in how they handle failure；require results in a Fatal Error；include results in only a warning and continue；