Waiting for a time (or counting down) to execute an event

Is there a function, or could somebody help me devise a way, to have a php script count down seconds, or wait for a specific system time to arrive, before continuing with a script? I'm working on a system that would do something every X number of seconds, and I'm trying to figure out if I can do it all in PHP or if I'm going to need some kind of back-end. tia, Mark