PHP referrer: How to make sure a request is actually coming from where it's supposed to come from

Question

I would like to put a link back to my site on other "approved" domains. When they click on the link it goes to a page that checks the referrer ($_SERVER['HTTP_REFERRER']) to make sure they came from a domain that is approved to have my link. This can be spoofed so how can I make sure the clicks are actually coming from the approved domains?

Answer 1

You can't do it. You can't prevent the referrer from being tempered with.

An alternative would be possible if there's collaboration between the several sites. For instance, the links in the other sites to yours could pass a token as a parameter in the URL that would be usable only once and which you could then validate.

Several validation strategies would be possible. Your site could contact the other site and ask it if the passed token is valid or you could employ a signature with the token acting as a nonce so you didn't have to contact the other site.