The Solution

When a user first visits our site we have to give them an identifier and ask them nicely to give this something back to us when they visit again. Then we tie whatever data we want to have persist across requests to this identifier. This is known as the session ID.

PHP has a number of built-in functions that implement this idea of creating a unique session id for each visitor and associating data with them. There is nothing particularly magical about these functions and you can easily come up with your own system for dealing with sessions.