public static function GetActiveFilter()
    {
        static $aFilter = 'x';
        if ('x' === $aFilter) {
            $oGlobal = TGlobal::instance();
            $aFilter = $oGlobal->GetUserData(MTShopArticleCatalogCore::URL_FILTER);
            if (!is_array($aFilter)) {
                $aFilter = array();
            }
            // now reduce filter list to valid filter fields
            $aValidFilterFields = TdbShop::GetValidFilterFields();
            $aValidFilter = array();
            foreach ($aValidFilterFields as $sField) {
                if (array_key_exists($sField, $aFilter)) {
                    $aValidFilter[$sField] = $aFilter[$sField];
                }
            }
            $aFilter = $aValidFilter;
        }

        return $aFilter;
    }