public function getString($command, $target = null, $value = null)
    {
        $result = $this->doExecute($command, $target, $value);

        if (!preg_match('/^OK,/', $result)) {
            throw new Exception("Unexpected response from Selenium server : ".$result);
        }

        return strlen($result) > 3 ? substr($result, 3) : '';
    }