This provides very simple access to PayPal's NVP API.
It is heavily inspired by Mahmoud Al-Qudsi's paypalnvp-php See https://github.com/NeoSmart/paypalnvp-php
PayPal NVP API: https://developer.paypal.com/webapps/developer/docs/classic/api/merchant/DoExpressCheckoutPayment_API_Operation_NVP/
For an implementation example, see payment.php