free topic software

I was recently asked:

I was wondering if you could give me some programming advice.  I'm working on 
a website for a professor at St. Mike's. She wants users to be able to post 
messages in response to topics.  I have some relational  database knowledge, 
know how to use php and MySQL.  What I was wondering is if there's a better 
way to store, retrieve and sort messages, maybe some free code out there for 
the taking.

I thought the list might have some ideas on free software for this.  Must 
exist.  Thanks.

(Might be of interest to the BH site as well...)

David