请教 curl 登陆 验证码的问题

请各位高手帮忙看看， 为什么会出线 验证码 变化的情况？ 该咋个修改啦<form id="form1" name="form1" method="post" action="__URL__/postGyyxDate" style="margin:0px">
请输入验证码:
<input type="text" name="gyyxSeccode" id="gyyxSeccode" value="" />
<input type="submit" name="sub" value="提交" style="float:left">
</form>
验证码:<img src="__URL__/getGyyxIntegralSecCode"  />
function postGyyxDate(){

session_start();

if($_SERVER["REQUEST_METHOD"]=="POST")
{
$array=array(
'txtUserName'=>'test250',
'txtUserPassword'=>'test250',
'__VIEWSTATE'=>'/wEPDwUJNjA4OTE3MjQwD2QWAgIBD2QWAgIDDw8WAh4HVmlzaWJsZWhkZGRunW2iOaIX8JGlJMrE8QahY/NE7Q==','__EVENTVALIDATION'=>'/wEWBQKl9caCBgKl1bKzCQK9wKW7DAL+geuuCwKC3IeGDILX0crrtL6gOrkuj/AYCJ1hNrJv','txtCaptchaCode'=>trim($_POST["gyyxSeccode"]),
);

}

$login_Submit_url  =  'http://account.gyyx.cn/Passport/EmbedLogin.aspx';

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL,$login_Submit_url);
curl_setopt($ch,CURLOPT_COOKIEFILE,$_SESSION['gyyx_cookie_file']); curl_setopt($ch,CURLOPT_REFERER,"http://www.gyyx.cn");
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $array);
curl_setopt($ch,CURLOPT_RETURNTRANSFER,1);
curl_setopt($ch, CURLOPT_HEADER, true);

$content = curl_exec($ch);
echo mb_convert_encoding($content, 'gb2312','utf-8'); curl_close($ch);

} function getGyyxIntegralSecCode(){

session_start();

$secCode_url  = 'http://account.gyyx.cn/CaptchValidateCode.ashx';

 $_SESSION['gyyx_cookie_file'] = tempnam('./temp','gyyx_cookie');

$ch = curl_init();

curl_setopt($ch,CURLOPT_REFERER,"http://www.gyyx.cn");

curl_setopt($ch, CURLOPT_URL,$secCode_url);//是要得到内容的地址  

curl_setopt($ch,CURLOPT_RETURNTRANSFER,false);

 
  curl_setopt($ch, CURLOPT_COOKIEJAR, $_SESSION['gyyx_cookie_file']);
if(strpos(strtolower($_SERVER['HTTP_USER_AGENT']),'msie')===false)
{
header("content-type:image/jpg");
}

curl_exec($ch);

curl_close($ch);

}里面的用户名和账号都是有效的 ，请高手帮忙看看 应该怎么样修改代码  第一次 curl取回的 验证码 
手动输入 post回去后 就变了 ，请各位高手指教下