Online Interaction between swf files

Hey guys. Im interested in knowing how to get 2 swf files to interact over a server. Lets just say there is a button on screen and you can click it on and off. I want anyone who has the swf loaded to be able to turn it on and off at will and when they do, it does on all the other uses computers as well.

I know actionscript pretty exstensivly and I started learning PHP and XML last week, im finding it pretty easy so far.

Any information/help would be greatly appreciated.

Thanks