include() from another sub-domain

Hello, I've searched the list and the manual and am still a bit confused. Here's what I'd like to do. We have a bunch of internal sites that all use the same username/password. One is already setup with a function to authenticate this to a database. However, each login page is coming from a different sub-domain, such as site1.mydomain.com. The login functions resides in a PHP file on site2.mydomain.com. So it does this: <? // This page on site1.mydomain.com include ("http://site2.mydomain.com/functions.php"); if (loginUser($username,$password)) { // do stuff here // redirect } else { ... } ?> Right now my local script doesn't find the function I'm trying to use. What am I missing here? Is it not possible to include a php file (and its functions and variables) across domains? - Greg