Dear Sir or Madam,
I've got quite the experience for this project because I've developed a browsergame myself.
So, for the security I'd check the common issues like MySQL injections and remote file inclusion. Of course there are other possible security holes like XSS attacks and co, but I'd also check them.
So as You can see, security's not a big deal to me.
But now to the important idea. As I said above, I once developed a browsergame myself, so I had the same problems you are facing now so I came up with a brilliant idea using cookies.
If You are interested in me just ping me so we can arrange the conditions to match our needs.
I'm looking forward to a great working relationship.
Best Regards