<img src="image.php" mce_src="image.php" onclick="this.src='image.php?timeamp='+ new Date().getTime();" title="看不清，点击换一张" alt="刷新验证码"/>

//image.php

<?php
ob_clean();
Header("Content-type: image/gif");
/*
* 初始化
*/
$border = 0; //是否要边框 1要:0不要
$how = 4; //验证码位数
$w = $how*25; //图片宽度
$h = 45; //图片高度
$fontsize = 15; //字体大小
$alpha = "abcdefghijkmnopqrstuvwxyz"; //验证码内容1:字母
$number = "023456789"; //验证码内容2:数字
$randcode = ""; //验证码字符串初始化
srand((double)microtime()*1000000); //初始化随机数种子

$im = ImageCreate($w,$h); //创建验证图片

/*
* 绘制基本框架
*/
$bgcolor = ImageColorAllocate($im, 255, 255, 255); //设置背景颜色
ImageFill($im, 0, 0, $bgcolor); //填充背景色
if($border)
{
    $black = ImageColorAllocate($im, 0, 0, 0); //设置边框颜色
    ImageRectangle($im, 0, 0, $w-1, $h-1, $black);//绘制边框
}

/*
* 逐位产生随机字符
*/
for($i=0; $i<$how; $i++)
{  
    $alpha_or_number = mt_rand(0, 1); //字母还是数字
    $str = $alpha_or_number ? $alpha : $number;
    $which = mt_rand(0, strlen($str)-1); //取哪个字符
    $code = substr($str, $which, 1); //取字符
    $j = !$i ? 4 : $j+25; //绘字符位置
    $color3 =

ImageColorAllocate($im, mt_rand(0,100), mt_rand(0,100), mt_rand(0,100)); //字符随即颜色
   

ImageChar($im, $fontsize, $j, 16, $code, $color3); //绘字符
    $randcode .= $code; //逐位加入验证码
}

/*
* 添加干扰
*/
for($i=0; $i<5; $i++)//绘背景干扰线
{  
    $color1 = ImageColorAllocate($im,mt_rand(0,255), mt_rand(0,255), mt_rand(0,255)); //干扰线颜色
    ImageArc($im, mt_rand(-5,$w),mt_rand(-5,$h), mt_rand(20,300), mt_rand(20,200), 55, 44, $color1); //干扰线
}  
for($i=0; $i<$how*40; $i++)//绘背景干扰点
{  
    $color2 = ImageColorAllocate($im, mt_rand(0,255), mt_rand(0,255), mt_rand(0,255)); //干扰点颜色
    ImageSetPixel($im, mt_rand(0,$w), mt_rand(0,$h),$color2); //干扰点
}

//把验证码字符串写入session
session_start();
$_SESSION['randcode'] = $randcode;

/*绘图结束*/

Imagegif($im );
ImageDestroy($im);
/*绘图结束*/
?>