PHP sessions stored in a Mongo database

When it comes to coding user based websites and web apps, one of the concepts you'll no doubt have to comprehend is handling user sessions. If you're using a server side language like PHP, session handling comes about fairly easily - simply declare a session_start() on every refresh, and store and retrieve values from the $_SESSION array. By default, PHP will read and write this session data using the filesystem, but what if you want to move this functionality to a database?