creating a threaded message system--sorting messages

This question might deviate from PHP into the domain of MySQL but I thought best to post here first. I'm building a message board system with PHP/MySQL. I'm trying to present the messages to the users in threaded order rather than flat. I'm having a lot of trouble figuring out how to sort the posts so they appear in the correct threaded order. I don't think I can do this purely with a SQL query. If it can be done this way, please suggest how and I'll take this question to the MySQL list. I think I have figured out the basic logic, I just have no idea how to translate it into code. Also, I may have the logic wrong. Anyhow this is what I have so far: relevant data structure loosely: post_id (unique, autoincrement, primary index) parent_id (if the post is a child, this field contains the post_id of its parent) ... 1) Query the database for all messages under a certain topic, sort by parent_id then post_id 2) Somehow resort the data so that each group of children is directly after their parent. Do this in order of ascending parent_id. Can this be done with usort() and some programatic logic/algorithm? How do you sort groups of items together rather than comparing each array element to the next array element (ie: sorting one item at a time)? Should this be done with a recursive algorithm? Anyone with experience writing code for this type of message board, or implementing existing code? Thanks for any help in advance. - Ben