[Calgon]

It's usually a PHP / MySQL script, which is called a "User Control Panel" or a "User Registration System", there's probably a few private releases of UCPs, there may be a few on this forum, but the registration scripts are highly dependent of which variables would be required for the account.

Try play with PHP and MySQL code and see what you can produce, there are several tutorials all over the internet, just search. Note: If you're going to be using MySQL, you'll need a plugin for it to interact with on your SA-MP server, check the plugins section.