Allowing access by referrer

yes, that;s true and I'm aware of this, but this is how the client wants to do this.

also, the IP option is not feasable as the site referring to me is open from everywhere to everyone, so there is no one IP or a group of IP's I can allow.

does someone have an idea where i can find an example of how to add a cookie to a session, and how to allow access by a cookie?

many thanks for the help.