restrict access to url address via referrer

[pc]

What would be the best way to restrict access to a page so it can be accessed only from a certain place. Yes I googled and there seems to be a few ways to achieve that. Bulletproof one I need.

[ecchi]

Depends how many allowed domains. If you only allow access from a small number of domains I'd simply go via a simple Perl if else loop (IE: If if referring URL is one of these redirect to your locked page, else redirect it to a sponsor or whatever).