If you've ever wanted a place for your visitors to just leave a quick and easy message on your site (without the hassle of registration/going through the forums/etc), then DevShed might just have an article you'd be interested in.

Tag boards enable users to leave a short message on your site without having to go through the trouble of registering. They are the new age guestbook, and apart from giving the visitors to your site a voice, they are sure to make your friends envious too.

Tag boards, in most cases, usually consist of a content area (normally an iframe) and an input area where users can put their message, name and a URL. [...] From a development point of view, they are actually rather simple to develop. In this article we will create a quick and easy tag board for any web site. We will be taking advantage of the PHP and MySQL technologies, and a basic knowledge of both is assumed.

They provide not only the whole database structure for you, but also the code that makes it tick (and what each part of the code does). It's a pretty light application, and is a great place to start for those out there getting used to PHP and how it all works - especially the ones getting to functions and database connectivity...