It will be a mix of PHP, MYSQL and JavaScript.
Getting the JavaScript to work exactly the same on different platforms (as it calculates the velocity/position of everything on each user's machine independantly, so a difference of 0.000001 could mean the difference between a collision being detected or not on one machine, and not on another during the same battle) is going to be the most tricky problem.
I could assign one client as the master, which would send the results of all of the calculations to the server, which then pushes them out to the other clients.
The gameplay will revolve around sending a series of commands to the tank, for example
f10;r+45;s1;
could mean
Foreward for ten seconds; Rotate 45 degrees clockwise; Shoot weapon one;
besides the basic battles, there could be flag capture, tank football, etc, but I'll get the basic game working first.