Global register variables are just straigtforward information fields that fetch the information from a field. Many forms use to use PHP variables such as $user_name to fetch the user name. Because of this very basic method, it was subject to many security holes, so many of these variables are either combined into PHp functions in management systems ( such as Wordpress get_the_post, which is a set of globals I believe ) or are no longer used. A good analogy would be like going to the post office to pick up mail. They could just ask your name and address, and then you take the mail. That is essentially what global functions are: filling out the form and taking the mail. Anyone can dupe this system and take your mail, or worse. What now happens is a system of validation, checking, and functions. So now you go to the post office, and it asks you to validate your name with an ID. Essentially a function to keep data from being abused. It would be in the form of more advanced functions and security. I hope this make a wee bit of sense.