Protecting Include remote files

Hi, I have two server running php. One of them connect with Oracle, and have been used for Php application for administration in my institution, and only a certain user can use it (let's call it server1). The second server is only a web server (server2). Now, I am creating an application in server1 (using database and stuff). I want to displayed to public, thus I include the application page in server2. So, in server2, I would have something like include ('http://server1.domain.com/blah.php3'); . My question is, can I protect that application in server1 (blah.php3) so that it cannot be included by anyone except by server2 ? I mean, if I know the URL address for the application in server1, I can use any server, serverX, to include that application in my page right? Is there anyway to avoid this? Thanks. Reuben D. Budiardja