[cnnamon]

I was planning on using php for backend and javascript+html5 for front end.

[Eraden]

I could say to propose you to use NodeJS instead but probably you already heard that. First at read this:

http://buildnewgames.com/real-time-multiplayer/

This will describe you complexity of real time client-server communication.

Start with login, go slowly to current game session state and updating it. Better make couple dummy games before you sit down on something really serious.