Game development approach

Hi there, I'm developing an online game (using PHP+MySQL, and being totally interfaced via web) which needs to have a "game cycle" running. For example, the player is flying a plane, so he sets the plane speed to 10%, then the game cycle needs to keep "moving the plane" forward (i.e. updating position), while the player doesn't order it a full stop. The point is how to implement this "game cycle"? I've done some research on the web and haven't found a feasible approach to this idea. Thanks in advance for any help. []s Alex