XML architecture question

Hi List, hope somebody here can help me out here. For a new page I'm designing I see one problem coming up. As I need to generate varius outputs such as HTML or PDF I think going over XML isn't a bad idea. I create one XSL for my output and change the XML for Customer A or B behind it. Now how do I make sure customer A doesn't see customer B's XML file? I could generate the XML for every time I need it or store it as a blob are some of the ideass that came in my mind. Has anybody an idea on how to solve this? Are solutions for this available? Customer A + B have to identify thru an authentication mechanism with PHP of course. thx Jabro