protected function getHelper($helperName)
    {
        if (array_key_exists($helperName, $this->helperClassInstances)) {
            $helperClassName = "Shopgate_Helper_" . $helperName;
            if (!isset($this->helperClassInstances[$helperClassName])) {
                $this->helperClassInstances[$helperClassName] = new $helperClassName();
            }

            return $this->helperClassInstances[$helperClassName];
        }
        throw new ShopgateLibraryException(
            "Helper function {$helperName} not found",
            ShopgateLibraryException::SHOPGATE_HELPER_FUNCTION_NOT_FOUND_EXCEPTION
        );
    }