Just a quick note this Saturday morning (well, okay, it's morning here). Anyway - PHPEverywhere has a new article posted that is an excerpt from an O'Reilly book that "discusses using PHP to integrate two web applications into a single interface. In the given example, the authors show how a technology news service based on RSS (RDF Site Summary) feeds was created."
I've been messing with XML of late - and I have to admit, I used to not think it was very useful, but trust me - couple it with the XML parsing that PHP already has and you have one very powerful tool for server-to-server communication.