I am currently doing a lot of work with the PayPal-PHP-SDK REST API, now at version 1.11.0. I have been using this since around October 2014, when it was at version 0.13.0 Before that I was using PayPal X.com.
I am implementing shopping cart and membership, and using payment methods of both credit card and paypal. I am using IPN. I am in the process of moving to webhooks. For the website implementation I am using PHP, JavaScript, JQuery, Ajax, HTML, CSS, and MySQL.
I am on the PayPal PHP-SDK github as kenjohnson https://github.com/paypal/PayPal-PHP-SDK. I have created 13 issues including one bug and one api-bug.
If you see a question or answer from me, hopefully this profile will give you an idea of my background.