PayPal Express Checkout easy integration.
1. Create object instance:
$paypal = new PaypalExpressCheckout($gateway);
2. Redirect user to PayPal to obtain charging permissions
$paypal->doExpressCheckout(123.45, 'Test service', 'inv123', 'USD', $resultData);
3. Perform payment, token and PayerID are being returned with GET response from PayPal
$paypal->doPayment($_GET['token'], $_GET['PayerID'], $resultData)
4. Perform refund based on transaction ID (returned from doPayment method)
$paypal->doRefund($transactionId, 'inv123', false, 0, 'USD', '', $resultData)
For more information see index.php file.
NOTE: You need to have Express Checkout enabled on your live account