This library is written in php 5.3 and uses namespaces.
use PayPalNVP\PayPalNVP,
PayPalNVP\Profile\ApiSignature,
...
or you can use class names with namespaces.
Every request needs profile, which is either PayPalNVP\ApiSignature, or PayPalNVP\ApiCertificate. Most of the users use ApiSignature.
$profile = new ApiSignature('username', 'password', 'signature');
Request are sent (and responses set) using PayPalNVP\PayPalNVP object. Which takes profile and environment as arguments. Environment has only static constructors and acts as an enum.
$paypal = new PayPalNVP($profile, Environment::SANDBOX());
The rest of the code depends on what request you want to make. Set express checkout would look like this:
/* items with prices */
$items = array();
$items[] = PaymentItem::getRequest('10');
$items[] = PaymentItem::getRequest('14');
/* payment */
$payment = Payment::getRequest($items);
/* set express checkout operation */
$setEC = SetExpressCheckout::newInstance($payment, "http://your_success_url", "http://your_cancel_url");
/* request is sent and response set on $setEC */
$paypal->setResponse($setEC);
/* now you can call getter for response */
$setECResponse = $setEC->getResponse();
/* or if you prefer array of all name value pairs */
$nvpArray = $setECResponse->getResponse();
/* to check if there are any errors with the submission */
$errors = $setECResponse->getErrors();
All operations are very similar, they use main paypal object. You need to create an instance of a request (PayPalNVP/Request package) and response is set using setResponse method on PayPalNVP. All requests have getResponse() method which returns appropriate response.