I uploaded that file and nothing changed. I put this string in the checkout_failure page (<?php var_dump($_SESSION); ?>) and got a very long error message. At the bottom it says
Checkout PayerID is missing.Express Checkout PayerID is missing.10419ErrorTransaction refused because of an invalid argument. See additional error messages for details.The PayerID value is invalid.10406Error63.01741654"
I've double checked my credentials and made sure the sandbox option is checked. Why is it not working? I've attached the rest of the error message after removing some of the sensitive info.