php 判断网页是否是utf8编码的方法

$encode = mb_detect_encoding($q, array('GB2312','GBK','UTF-8'));
echo $encode."<br/>";
if($encode=="GB2312")
{
    $q = iconv("GBK","UTF-8",$q);
}
else if($encode=="GBK")
{
    $q = iconv("GBK","UTF-8",$q);
}
else if($encode=="EUC-CN")
{
    $q = iconv("GBK","UTF-8",$q);
}
else//CP936
{
    //$q = iconv("GB2312","UTF-8",$q);
}