Periodic Actions in PHP?

Hi there, I'm going to create a small chat script with PHP. The messages you want others to see will be added to a flat file (I.E. TXT file), and read and displayed by PHP. However, I want this reading and displaying to be periodic. This means that I want PHP to check the file for new lines every,say, fifteen seconds. How may I do that? I have been unable to find any function that acts like a timer. Thanks! -- --- Contact info: Skype: parham-d MSN: fire_lizard16 at hotmail dot com email: parham90 at GMail dot com