Javascript Src and HTTP_REFERER

Hi, I have a web page (say page1) which refers to a php within javascript tag as: <script type="text/javascript" src="http://mysite.com/javascript_dump.php"></script> As you can see, this calls a php file which dumps javascript. The problem here is, within this php I couldn't get which page it is called from. Basically, if it comes from affiliate site 1, I will show some content, and if it comes from affiliate site 2, I will show some other content. And, the thing is, within this javascript_dump.php, HTTP_REFERER shows nothing, because it gives you the referer of the html page (page1) that contains this <script> tag (which might be something irrelevant, i.e. google), and remote_addr gives you the IP address of the user. I know that I can add a simple query parameter to specify the source web page such as: javascript_dump.php?siteid=xyz, but this can be faked very easily, and anyone can introduce themselves as any affiliate. Is there a way of getting (within the php) the site this php was called from in a reasonably secure way? Any ideas/suggestions? Thanks..