Hiya,
It is a much simpler implementation to use javascript, to be honest.
You can see a working example of a single click refreshing two frames at:
http://warburton.comwww2.dsvr.co.uk/salessols
You could also do it using PHP if you always load the whole frameset back in each time you make a choice, but have a single database table telling you which pageid you requested and which html page goes into each part of the frameset.
I've just posted asking for a small amount of help on what I think is similar, but my implementation is actually working, so you could look at that post if you need ideas.
Hope this helps
Richard