snoopy 伪装代理

我用Snoopy类的时候，伪装代理如下：

$snoopy->agent = "(compatible; MSIE 4.01; MSN 2.5; AOL 4.0; Windows 98)"

可是我输出$_SERVER["HTTP_USER_AGENT"]
代理还是我浏览器的代理，根本就没变？请各位给点意见

$snoopy->agent = "(compatible; MSIE 4.01; MSN 2.5; AOL 4.0; Windows 98)";
$snoopy->fetch('xxx.php');
echo $snoopy->results;

在xxx.php里写
echo $_SERVER["HTTP_USER_AGENT"];