On PHPMaster.com today there's a new post showing you how to write custom session handlers - in their case, a database-based option that can span across multiple servers/services.

Sessions are a tool which helps the web programmer overcome the stateless nature of the internet. You can use them to build shopping carts, monitor visits to a website, and even track how a user navigates through your application. PHP’s default session handling behavior can provide all you need in most cases, but there may be times when you want to expand the functionality and store session data differently. This article will show you how the default functionality works and then goes on to show you how override it to provide a custom solution.

They introduce how sessions are stored normally (based on the save path) and what the serialized contents of it would look like. They show you how to take this, normally stored on the local file system, and change it to be written to a "session" table in a MySQL database (via PDO). It includes reading, writing, updating and some garbage collection to clean out old values from the table.