php 验证码

checkcode.php
====================
<?php
session_start();
$funcs = array('imagecreatetruecolor','imagecolorallocate','imagefill','imageline','imagedestroy','imagecolorallocatealpha','imageellipse','imagepng');
if(!function_exists('ob_gzhandler'))
    ob_clean();
 //create captcha
 $consts = 'cdfgkmnpqrstwxyz23456';
 $vowels = 'aek23456789';
 for ($x = 0; $x < 6; $x++)
 {
  $const[$x] = substr($consts, mt_rand(0,strlen($consts)-1),1);
  $vow[$x] = substr($vowels, mt_rand(0,strlen($vowels)-1),1);
 }
 $radomstring = $const[0] . $vow[0] .$const[2] . $const[1] . $vow[1] . $const[3] . $vow[3] . $const[4];
        $_SESSION['checkcode'] = $string = substr($radomstring,0,4); //only display 4 str
        //set up image, the first number is the width and the second is the height
 $imageX = strlen($radomstring)*8; //the image width
 $imageY = 20;      //the image height
 $im = imagecreatetruecolor($imageX,$imageY);
 //creates two variables to store color
 $background = imagecolorallocate($im, rand(180, 250), rand(180, 250), rand(180, 250));
 $foregroundArr = array(imagecolorallocate($im, rand(0, 20), rand(0, 20), rand(0, 20)),
         imagecolorallocate($im, rand(0, 20), rand(0, 10), rand(245, 255)),
         imagecolorallocate($im, rand(245, 255), rand(0, 20), rand(0, 10)),
         imagecolorallocate($im, rand(245, 255), rand(0, 20), rand(245, 255)));
 $foreground2 = imagecolorallocatealpha($im, rand(20, 100), rand(20, 100), rand(20, 100),80);
 $middleground = imagecolorallocate($im, rand(200, 160), rand(200, 160), rand(200, 160));
 $middleground2 = imagecolorallocatealpha($im, rand(180, 140), rand(180, 14