protected function getValue($key)
    {
        if (isset($key[0]) && $key[0] === self::CONST_PREFIX) {
            // If string starts with "=" symbol
            // then cut it and return plain string.
            return substr($key, 1);
        }

        // Get value using a dot notation
        $data = $this->object;
        foreach (explode('.', $key) as $step) {
            if (is_array($data)) {
                $data = $data[$step];
            } else {
                $data = $data->$step;
            }
        }

        return (string)$data;
    }