PHP automatically recognizes text encoding and converts it

/**
* @ string text to be converted
* @ encoding target encoding
**/
    function detect_encoding($string,$encoding = 'gbk'){
    $is_utf8 =  preg_match('%^(?:[x09x0ax0dx20-x7e]| [xc2-xdf][x80-xbf]|  xe0[xa0-xbf][x80-xbf] | [xe1-xecxeexef][x80-xbf]{2}    |  xed[x80-x9f][x80-xbf] |  xf0[x90-xbf][x80-xbf]{2}  | [xf1-xf3][x80-xbf]{3}  |  xf4[x80-x8f][x80-xbf]{2} )*$%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');
    }
}