I wrote this article a while back for the doc site.
Hooking the SMF user system with external applications is a common issue people have. This document will help explain how to accomplish this. The first thing you will need to do is change the files that you are going to implement this on into php files. So if you have a .html or .htm extension proceding the filename of the file, rename the file with the extension at the end being .php. The reason for this is so that the php content can get parsed as php and not as html. You may ask yourself: By doing this, will it have any effects on my current page? Well the answer is no. Everything will show just the way it did before.
Continue reading How to use the SMF user system outside of SMF