I tried doing this (it's a surprisingly hard task to accomplish). There are two methods you can do:
1) Record their last-update time (when they got their last turn) and go from there
2) Use a cronjob to run a script that updates everyone (much easier!)
If you're using the first method, just record it some where, and then compare. Take the current time minus that number's time and then divide by the number of minutes you want between each turn. The end result is how many turns they need. You can then give them this many turns.
The latter method, the easier of the two, is.. much easier. Crontab is a program for Linux (there are ports for about every OS known to man) that will run a certain task at a specific time, including PHP scripts. All you need to do is make a PHP script that gives everyone one turn, and run the file every x minutes.
