一个PHP验证码类代码分享(已封装成类)

<?php 
session_start(); 
Header("Content-type: image/gif"); 
class SecurityCode 
{ 
private $codes = ''; 
function __construct() 
{ 
$code = '0-1-2-3-4-5-6-7-8-9-A-B-C-D-E-F-G-H-I-J-K-L-M-N-O-P-Q-R-S-T-U-V-W-X-Y-Z'; 
$codeArray = explode('-',$code); 
shuffle($codeArray); 
$this->codes = implode('',array_slice($codeArray,0,4)); 
} 
public function CreateImg() 
{ 
$_SESSION['check_pic'] = $this->codes; 
$img = imagecreate(70,25); 
imagecolorallocate($img,222,222,222); 
$testcolor1 = imagecolorallocate($img,255,0,0); 
$testcolor2 = imagecolorallocate($img,51,51,51); 
$testcolor3 = imagecolorallocate($img,0,0,255); 
$testcolor4 = imagecolorallocate($img,255,0,255); 
for ($i = 0; $i < 4; $i++) 
{ 
imagestring($img,rand(5,6),8 + $i * 15,rand(2,8),$this->codes[$i],rand(1,4)); 
} 
imagegif($img); 
} 
} 
$code = new SecurityCode(); 
$code->CreateImg(); 
$code = NULL; 
?>