PHPBuilder.com has posted a tutorial showing you how to interact with the PayPal API via your PHP application using their own PHP-SDK.
PayPal recently introduced a new RESTful API that is more convenient and more powerful than the previous version. In this article, I will show you how to integrate your PHP application with the new PayPal API.
They start with a summary of the PayPal API and how to get the SDK loaded and ready to use (either through Composer or manually). The tutorial walks you through the authorization process (OAuth) and the code you'll need to make it happen. They also show you how to create transaction (including currency type and description) after the items have been submitted. There's also some code showing you how to get the current status of the payment once it has been submitted.