Problems with document() [PHP5, libxslt]

I'm trying to use the document() function to grab some XML nodes from a
web application I've built. I'm grabbing from two places: a news page
(insecure) and a user's profile page (secure).

I'm having no problems with the insecure news page, everything is being
grabbed and used correctly.

The secure user's profile page, however, is another matter. The webapp
uses cookies to maintain the user's session, and keeps them
authenticated. Accessing the page without being authenticated (having
the cookie set) forwards the user to a login page. This is what happens
when I grab the page using XSL's document() function.

I've tried to "pick-up" the session by passing the session information
in the GET parameters, and I'm getting a strange error: It seems that
the processor is going into a loop.

I'm pretty sure sessions aren't the problem though, as a new session is
started when ever the webapp is accessed. New sessions are started for
the news page and the login page, and things run fine.