Setting up a registration code...

I am taking a look at some software and am planning on using Zend Encoder to distribute my product with but I am looking to have registration numbers. Has anyone managed to set something up like this: Person installs product and runs it the first time, this then queries a website/database to make sure it is a valid code or that it hasn;t been used already? Or if there are other similar ways that would be cool to know as well. Jeff