paypal API fix needed (urgent) database injection upon return

hello there , I need a freelancer to explain to me a php file that handles a return after a Paypal payment using the Paypal API. Upon user's return to the WordPress website, I need the database to be injected so some changes in regards to the user's role will be modified. I will give you the code for you to study, your first work will be to be able to understand the functions of this file and then work with me so the database tables to be updated as needed.