php显示动态验证码

需要安装的库文件:

gd-2.0.33-9.4.el5_1.1

php-gd-5.1.6-23.el5  这个经常忘

gd-devel-2.0.33-9.4.el5_1.1

freetype-devel-2.2.1-19.el5

freetype-2.2.1-19.el5

下面是一个动态验证码的php文件:

<?php
Header("Content-type: image/PNG");
//采用session的方式传递
session_start();
$_SESSION['check_pic']='';

//生成画布
$im = @imagecreatetruecolor(60,20) or die('create error');

$black = ImageColorAllocate($im, 0,0,0);
$white = ImageColorAllocate($im, 255,255,255);
$gray = ImageColorAllocate($im, 200,200,200);
//生成验证码数字
$check_num = rand(1000,9999);
$_SESSION['check_pic']=$check_num;

imagefill($im,0,0,$gray);
$text_color = imagecolorallocate($im, 0, 0, 255);
imagestring($im, 5, 10, 2, $check_num, $text_color);
//增加干扰点
for($i=0;$i<255;$i++){
        $randcolor = ImageColorAllocate($im,rand(0,255),rand(0,255),rand(0,255));
        imagesetpixel($im, rand(0,$i), rand(0,$i), $randcolor);
}

imagepng($im);
imagedestroy($im);
?>

显示php验证码：

<td><img src='../checkcode.php' align='absmiddle'>
<input type='text' size=5 name='typecode'></td>