php 数学运算验证码实现代码

<?php 
//------------------------------------- 
// 文件说明：数学运算验证码 
// 文件作者：Jesse Lee 
// 最后更新：2008-09-07 
//------------------------------------- session_start(); 
$sessionvar = 'vdcode'; //Session变量名称 
$width = 150; //图像宽度 
$height = 20; //图像高度 
$operator = '+-*'; //运算符 
$code = array(); 
$code[] = mt_rand(1,9); 
$code[] = $operator{mt_rand(0,2)}; 
$code[] = mt_rand(1,9); 
$code[] = $operator{mt_rand(0,2)}; 
$code[] = mt_rand(1,9); 
$codestr = implode('',$code); 
eval("\$result = ".implode('',$code).";"); 
$code[] = '='; 
$_SESSION[$sessionvar] = $result; 
$img = ImageCreate($width,$height); 
ImageColorAllocate($img, mt_rand(230,250), mt_rand(230,250), mt_rand(230,250)); 
$color = ImageColorAllocate($img, 0, 0, 0); 
$offset = 0; 
foreach ($code as $char) { 
$offset += 20; 
$txtcolor = ImageColorAllocate($img, mt_rand(0,255), mt_rand(0,150), mt_rand(0,255)); 
ImageChar($img, mt_rand(3,5), $offset, mt_rand(1,5), $char, $txtcolor); 
} 
for ($i=0; $i<100; $i++) { 
$pxcolor = ImageColorAllocate($img, mt_rand(0,255), mt_rand(0,255), mt_rand(0,255)); 
ImageSetPixel($img, mt_rand(0,$width), mt_rand(0,$height), $pxcolor); 
} 
header('Content-type: image/png'); 
ImagePng($img); 
?>