server - Synchronize turn based browser game

I'm writing a browser game in php and Sql. I'm also using Javascript - Ajax and Mysql.

I'm stack on the battle system because I want to Synchronize the turns of the players in the battle.

What I doing is to put two players on a battle. So from the first turn a countDown of 60sec will start. What I am thinking is to use a server function to check the countdown, not to the client side. This because I think that in that case a bad synchronization will came, instead if it is the server to count down.

But.. how can I let the server do all this stuff?