public function getSortOrder()
    {
        $sortOrder = isset($this->requestParams[self::PARAM_SORT_ORDER]) ?
            $this->requestParams[self::PARAM_SORT_ORDER] : self::DEFAULT_SORT_ORDER;
        $sortOrder = mb_strtolower($sortOrder);

        if (!in_array($sortOrder, ['asc', 'desc'])) {
            $sortOrder = self::DEFAULT_SORT_ORDER;
        }

        return $sortOrder;
    }