But you could make an own link for every from url.
Example: http://example.com?auth=holasite
In this example your site is: example.com
If somebody open that link it's give you the holasite value for the auth variable.
Then just $_GET['auth'] and you have the variable. But you should have a database to store it, and to authorize.
Like: $holasite = http://holasite.com (You could use mysql too..)
And just match it, and you have the url.
This method is a little bit more complicated, but it works. This method is good for a referral system authentication. But where is the site name, you should write an id, and works with that id.