php基本的验证码

<?php
/*
 * Created on 2014-12-8
 *
 * Tge the templats erated file go to
 * indow - Prefer- P Code Templates
 */
 session_start();

$rand='';
/*设置字符*/
 for($i=0;$i<4;$i++){
 	$rand.=dechex(rand(1,15));
 }
 $_SESSION['check_pic']=$rand;
 /*设置画布大小*/
 $img=imagecreatetruecolor(100,30);
/*设置颜色*/
$bg=imagecolorallocate($img,0,0,0);
$te=imagecolorallocate($img,255,255,255);
/*将字符串写入图像*/
imagestring($img,rand(1,6),rand(1,70),rand(1,10),$rand,$te);/*图片，字体，x轴距离，y轴距离，写入的文字，写入的颜色*/
/*输出图像*/
header("Content-type: image/jpeg");
imagejpeg($img);
?>