此函数是PHP中哪方面的学问，其功能是何

function js_unescape($str)    

{    

    $ret = '';    

    $len = strlen($str);    

    for ($i = 0; $i < $len; $i++)    

    {    

        if ($str[$i] == '%' && $str[$i+1] == 'u')    

        {    

            $val = hexdec(substr($str, $i+2, 4));    

            if ($val < 0x7f) $ret .= chr($val);    

            else if($val < 0x800) $ret .= chr(0xc0|($val>>6)).chr(0x80|($val&0x3f));    

            else $ret .= chr(0xe0|($val>>12)).chr(0x80|(($val>>6)&0x3f)).chr(0x80|($val&0x3f));    

            $i += 5;    

        }    

        else if ($str[$i] == '%')    

        {    

            $ret .= urldecode(substr($str, $i, 3));    

            $i += 2;    

        }    

        else $ret .= $str[$i];    

    }    

    return $ret;    

}