include('/path/to/lib/payments.php');
$payments = new PHP_Payments;
$config = Payment_Utility::load('config', '/path/to/config/file'); //You can do this or you can make this an array. Config file is recommended since configuration needs differ by gateway.
//Make the call
$response = $payments->payment_action('gateway_name', $params, $config);
There are two types of responses returned, local responses and gateway responses. If a method is not supported, required params are missing, a gateway does not exist, etc., a local response will be returned. This prevents the transaction from being sent to the gateway and the gateway telling you 3 seconds later there is something wrong with your request.
Accessing response properties looks like this:
$status = $response->status;
Gateway responses will usually have a full response from the gateway, and on failure a 'reason' property in the details object:
$gatway_response = $response->details; $failure_reason = $response->reason;