easiest way is to use frames/iframes since it's just basic HTML. Though nothing is ever easy without a compromise in that search engines HATE frames and iframes!
The other way is to use AJAX, much more user friendly, but again, search engines won't find the content loaded by AJAX very easily.
In PHP, you just need to pass a value back to your menu object. COuld be done in various ways, either via URL or better still, via the session