Mike Bernat has posted this tutorial that shows the creation and use of a simple registration script as created in PHP.
The most useful and almost necessary feature of any interactive website is a registration form. A way to give users access to features while keeping out the riff-raff. While a registration script can be the perfect time for an amateur to grab the bull by the horns it can also leave lots of pitfalls for a more experienced programmer.
He sets up the database first (for the user's information) followed by the simple HTML form to get the user's information. There's some error checking (if the value is empty or not) before he pushes it into the database, but not much checking outside of that (not even escaping to prevent a SQL injection). It's a very basic script that's a decent starting place, but needs work to be really used for user's information.