php 不刷新页面，点击刷新验证码

code.php

<?php
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*15; //图片宽度
$h = 20; //图片高度
$fontsize = 6; //字体大小
$alpha = "abcdefghjkmnpqrstuvwxyz"; //验证码内容1:字母
$number = "23456789"; //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+15; //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, 3, $code, $color3); //绘字符
    $randcode .= $code; //逐位加入验证码字符串
}


/*
* 添加干扰
*/

for($i=0; $i<10; $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*/
?>

<?php 
/* 
* 
* 
*/ 
session_start(); 
if($_POST[sub]){ 
    //if(!empty($_POST[gd_pic])){
//判断验证码是否相同 
if($_POST[gd_pic]==$_SESSION[randcode]){ 
echo "验证成功！"; 
}else{ 
echo "验证码错误"; 
} 
} 
?> 
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <title>验证码测试</title>
        
    <script>  
         function formSubmit(){   
             document.actionForm.action = "login.php";   
             document.actionForm.submit();   
         } 
         </script>
</head>

<body>
<form name="actionForm" action="login.php" method="POST"> 


验证码：<input type="text" name="gd_pic"/><img src="code.php" onClick="this.src='code.php'"/><br> 

<input type="submit" name="sub" value="submit"/> 
<a href="javascript:formSubmit()" name="sub">发布</a>
</form> 
</body>
</html>