PHP socket automatically shuts down after ? hours of idling?

Hi all, here is another question that made me confused. I'm not sure if it's a bug or what, but I can't find an answer from the search engines. I've made a chat server using php socket features, and it works pretty well as I've expected. However, there is a weird thing happening: The server shuts down automatically after about 3 hours of idling (no connections). I've set the php execution time limit to 0, and running this script from shell in the background of CentOS 5.3. Can't think of any other reason causing this strange behavior. Any hint? Thx!