Listen for file events in PHP - sort of like select() works in UNIX

I'm implementing a small chat system for my website.

The user se开发者_开发知识库nds a message to a PHP webpage... on the server this data is saved to a simple file. I read the file and WRITE some other message.

At this point I want the php web process to display the file change to the user.

So how do I listen for file change with PHP?