It is a framework developed in PHP5 that integrates easily with the Website Payments Pro API from Paypal. Currently it supports 4 major operations available from the Website Payments Pro SOAP 1.1 API. It has taken advantage of the built-in SOAP extension available natively in PHP5 to generate the requests and interprets the responses to and from the Website Payments Pro SOAP-based web service.
Current operations supported include: DoDirectPayment, SetExpressCheckout, GetExpressCheckoutDetails and DoExpressCheckoutPayment and you can get more information about other functionality from the project's documentation. Be sure to check out the sample script and the full SourceForge page for even more Paypal interface goodness.