public function permalink($string, $delimiter = '-', $option = PINYIN_DEFAULT)
    {
        if (\is_int($delimiter)) {
            list($option, $delimiter) = array($delimiter, '-');
        }

        if (!in_array($delimiter, array('_', '-', '.', ''), true)) {
            throw new InvalidArgumentException("Delimiter must be one of: '_', '-', '', '.'.");
        }

        return implode($delimiter, $this->convert($string, $option | \PINYIN_KEEP_NUMBER | \PINYIN_KEEP_ENGLISH));
    }