protected function currencyStringToNumeric($currency)
    {
        if (!in_array(strtolower($currency), array_keys(self::$currencyCodes))) {
            throw new HpsArgumentException(
                'Currency is not supported',
                HpsExceptionCodes::INVALID_CURRENCY
            );
        }
        return self::$currencyCodes[strtolower($currency)];
    }