I have a main product that is required to be purchased (or submit proof of purchase) before buying other products.
I have the "proof of purchase" requirement satisfied by submitting a code from your purchase receipt that stores a session variable $_SESSION['purchased'] = true. Then if this variable is true you can proceed to the all products page.
If they don't have a proof of purchase, How would I set this variable based on the (addition to the cart) of Product XYZ? Or is there a better way you can think of to accomplish this functionality?
I do realize that people may remove it from the cart after having placed it there, but the client understands that possibility.