Google auth and Paypal back-end

I am setting up a website and need help with google auth and paypal. The google login authentication is already working and paypal is not the problem either. My site will sell video courses, Udemy type and I want that when a person validates on my site and buys a course, the next time they access it, they don't have to buy again the course they already bought. To do this, I think I need to log authentication into a database and tell them which courses that user has already paid for so that the next time they access it they are not charged again.

The site is in HTML and ready to be launched. I only need the Back-End, which I prefer to be done in PHP with laravel for example.

I just need this development. as I said the Front-End its done!