Zebra_Session
A wrapper for PHP’s default session handler, using MySQL for storage
...The library is also a solution for applications that are scaled across multiple web servers (using a load balancer or a round-robin DNS) and where the user's session data needs to be available. Storing sessions in a database makes them available to all of the servers!
Zebra_Session's code is heavily commented and generates no warnings/errors/notices when PHP's error reporting level is set to E_ALL.
Starting with version 2.0, Zebra_Session implements row locks, ensuring that data is correctly handled in a scenario with multiple concurrent AJAX requests.