Remote authentication with PHP

[Anon]

I was wondering if anyone could help me out with a remote authentication problem in PHP.

I have an external partner that is linking to my pages. When user log on to this external sites they get a session id on their site, which I have to authenticate when they reach my site.

I have to ask the external partner with this link: http://url.for.partner/file?authenticate_session_id=xxxxxxx

From this I am supposed to receive an XML response from which I have to pull out a username, etc.

Does anyone know how this can be done.

Thanks guys.

Marius