public function encode($string)
    {
        $ret = '';
        for ($i = 0, $c = strlen($string); $i < $c; $i++) {
            if ($string[$i] !== '%' && !isset($this->preserve[$int = ord($string[$i])])) {
                $ret .= '%' . sprintf('%02X', $int);
            } else {
                $ret .= $string[$i];
            }
        }
        return $ret;
    }