php验证码读取问题 - PHP / 基础编程

做了一个验证码的程序，用number.php来生成验证码，代码如下：
PHP code:
session_start();
function randomkeys($length)
{
$pattern = '1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
for($i=0; $i<$length; $i++)
{
$key .= $pattern{mt_rand(0,61)};
}
return $key;
}


//$str = randomkeys(6);//输入的文字
$_SESSION['v']=randomkeys(6);
header("Content-type: image/jpeg");
$im = imagecreate(100, 30) or die("Cannot Initialize new GD image stream");//图片大小

//$str=iconv("gb2312","UTF-8",$str);
$_SESSION['v']=iconv("gb2312","UTF-8",$_SESSION['v']);
for($i=0;$i<200;$i++) //加入干扰象素
{
$clr = ImageColorallocate($im,rand(0,255),rand(0,255),rand(0,255));
imagesetpixel($im, rand()%100 , rand()%50 , $clr);
}

$black = imagecolorallocate($im, 0, 0, 0);
$fnt = "ARIAL.TTF"; //字体文件
ImageTTFText($im, 15, 0, 10, 20, $black, $fnt, $_SESSION['v']);
imagejpeg($im);
imagedestroy($im);



然后用refresh.php实现刷新，refresh.php代码如下：
PHP code:
<script language="JavaScript">
<!--
function change(id){
document.getElementById(id).src ='number.