pretty hard to understand what you mean, but most likely you need to have a file that will generate an XML document and output it depending on what user is using the script... it will be simly a matter of logic and such..
Yes i think sessions can be the solution.
I think the Flash here is just for the presentation part !?
1. You can store all the users' input in the same xml file, separating them using their sessions' ids.
2. Creating a temp xml file which name would be the users' session id. E.g.: 0accf10db6340c506caf0860510b8d01.xml.
3. Using another storage way (E.g.: MySQL), could be a great way to handle this. Using session as well.
pmaonline, tell us where/in what will be Flash and PHP involved in this scene.
PM me if needed as i'm portuguese too