[PHP] bulletin board algorithm in php - how to write one?

[Phil Schwarzmann]

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!!