php自动识别文字编码并转换

/**
* @ string 需要转换的文字
* @ encoding 目标编码
**/
    function detect_encoding($string,$encoding = 'gbk'){
    $is_utf8 =  preg_match('%^(?:[\x09\x0a\x0d\x20-\x7e]| [\xc2-\xdf][\x80-\xbf]|  \xe0[\xa0-\xbf][\x80-\xbf] | [\xe1-\xec\xee\xef][\x80-\xbf]{2}    |  \xed[\x80-\x9f][\x80-\xbf] |  \xf0[\x90-\xbf][\x80-\xbf]{2}  | [\xf1-\xf3][\x80-\xbf]{3}  |  \xf4[\x80-\x8f][\x80-\xbf]{2} )*$%xs', $string);
    if($is_utf8 && $encoding == 'utf8'){
        return $string;
    }elseif($is_utf8){
        return mb_convert_encoding($string, $encoding, "utf-8");
    }else{
        return mb_convert_encoding($string, $encoding, 'gbk,gb2312,big5');
    }
}