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);
}