private function convertString($string)
    {
        // chop away beggining and ending quotes
        $string = substr($string, 1, -1);
        if (strlen($string) > 255) {
            throw new WriterException('String is too long');
        }

        return pack('C', $this->ptg['ptgStr']) . StringHelper::UTF8toBIFF8UnicodeShort($string);
    }