public function getRequiredFields()
{
$fields = ['AMOUNT', 'CURRENCY', 'ORDER', 'MERCH_NAME', 'MERCH_URL', 'MERCH_GMT', 'COUNTRY', 'BRANDS', 'DESC', 'NONCE', 'TRTYPE'];
if ($this->parameters->containsKey('P_SIGN')) {
$fields[] = 'NONCE';
}
return $fields;
}
