For reasons that will soon become apparent, creating a messageboard is often a stumbling block for those learning PHP and so they do not progress on to more difficult challenges. This is a shame, because I consider messageboards the number one area where you can flex your creative muscle - there are dozens of cool features you can program into guestbooks to push your learning curve.
I do not think there is a messageboard system that currently exists that is perfect - people are always suggesting ideas for improvements, which means it is a really fertile area to get into. Even if you plan to use an off-the-shelf messageboard system such as phpBB, you should still learn how to make a simple one yourself as it will add another feather to your hat.
For this example, we're going to create a fairly complicated messageboard system. This might seem like overkill - after all, a messageboard is just there to send messages to and from members! However, the reason I have chosen to add a lot of information here about various features is because most if not all of it is applicable entirely in other projects you will do; we're going to be creating a fairly comprehensive solution here, and it will greatly help you if you can follow through all the way.