public function getSortType()
    {
        $sortType = isset($this->requestParams[self::PARAM_SORT_TYPE]) ?
            $this->requestParams[self::PARAM_SORT_TYPE] : self::DEFAULT_SORT_TYPE;
        $sortType= mb_strtolower($sortType);

        if(!in_array($sortType, ['string', 'numeric'])) {
            $sortType = self::DEFAULT_SORT_TYPE;
        }

        return $sortType;
    }