Whatever the XML is that you would store the session ID in, it would still have to somehow be transmitted from the server to the browser with each session-controlled page, and the browser would somehow have to send it back to the server whenever it requests a page.
All that currently gets transferred with a session cookie is one string of randomly generated characters for the session ID. Or if cookies are not enabled and if your PHP configuration allows for it, it will be transmitted via the URL or as a form field if applicable.
So now my question to you is, how would XML fit into this? XML is just a text mark-up specification. Sure, you could put the session ID into an XML-structured text string or file, but then what? That XML text would still have to be transmitted back and forth between the browser and the server. And where would the browser store it and then how would it transmit it back to the server? The only way I can think of is via a cookie, which therefore gains you nothing, but ends up requiring more data to be transmitted since now the same session ID string has to wrapped up in XML mark-up.
If you still consider this off-topic, then you'll need to find another way to explain to me what you are after that will finally make the light bulb go on over my head.