How to know when a user to a referred site signs up with or purchases from them?

I am in the process of developing a PHP application that awards users for supporting indie games. Once my website confirms an indie purchase, it will unlock certain VIP areas of my site.

Fo开发者_运维问答r example, I would have a link on my website to Minecraft's registration page. Once my referred user signs up and pays for his copy of Minecraft, is there any way for my site to instantly be updated that the user successfully bought a copy?

This isn't specific to Minecraft either. I would need to include many links to other indie developers' purchase pages.

I'm at a loss to where to begin on this, nor even sure if it's technically possible.

One way of doing this is via an API whereby the purchase is made on your site and you communicate with Minecraft on the backend.

Alternatively, affiliate networks like Commission Junction do the hard work of checking where the user came from and whether or not they buy the product.

However, it doesn't look like Minecraft offer either solution, so your only other option is contacting them directly and asking them if they want some kind of marketing partnership with you.

Yes, just ask the user. "Did you buy Minecraft? [yes] [no]"

Isn't it obvious that you need to get a response from the minecraft site to acknowledge a purchase?