PHP: Let user download purchased file ONLY

Question

I am forseeing a problem with allowing customers who purchase some content from me via PayPal. I will offer multiple, intangible goods. When someone completes their purchase for

Answer 1

When they make the payment, store the ID of the download available to them, and a random hash - both in the payment table. Use that hash to then get the ID. The hash should then never relate to a specific product, but instead to a payment.