PHP判断变量内容是什么编码

function convToUtf8($str) {
    if( mb_detect_encoding($str,"UTF-8, ISO-8859-1, GBK")!="UTF-8" ) {//判断是否不是UTF-8编码，如果不是UTF-8编码，则转换为UTF-8编码
        return  iconv("gbk","utf-8",$str);
    } else {
        return $str;
    }
}