Change this:
public function checkValidity( \IPS\nexus\Money $amount, \IPS\GeoLocation $billingAddress = NULL, \IPS\nexus\Customer $customer = NULL )
{
To:
/**
* Check the gateway can process this...
*
* @param $amount \IPS\nexus\Money The amount
* @param $billingAddress \IPS\GeoLocation|NULL The billing address, which may be NULL if one if not provided
* @param $customer \IPS\nexus\Customer The customer (Default NULL value is for backwards compatibility - it should always be provided.)
* @param array $recurrings Details about recurring costs
* @return bool
*/
public function checkValidity( \IPS\nexus\Money $amount, \IPS\GeoLocation $billingAddress = NULL, \IPS\nexus\Customer $customer = NULL, $recurrings = array() )
{