I decided to have a go at writing a bare-bones discussion board.
I've managed to get it so that you can list the posts and add posts but I cannot get the reply to work at all.
The idea is that when you click on 'reply', the current message id is passed on and becomes the parent message id. It all looks fine to me but the parent message id is always zero (which should only be the case when you start a new conversation).
I've bunged it up <a href="http://www.carlos-arredondo.f2s.com">here</a> for you to look at.
The source:
<a href="www.carlos-arredondo.f2s.com/index.txt">index.php3</a><b>
<a href="www.carlos-arredondo.f2s.com/add_con.txt">add_con.php3</a><b>
<a href="www.carlos-arredondo.f2s.com/reply_to.txt">reply_to.php3</a><p>
Many thanks.