bulletin board algorithm in php - how to write one?

I want to write a simple bulletin board in PHP. I want to use the typical parent-child (it's sorted by both date AND thread) type of bulletin board you see often on other websites. But I just can't figure out how to write some simple code to do this and keep track of all the threads. Any ideas? I don't even need example code, just if someone could quickly explain in plain english how to do it. Thanks!!