[Note: safely skip the descriptive part and go directly to the tool if you already know how PHP does session handling.] Sessions are a great feature as they allow developers to store sensitive data for a limited amount of time (the session lifetime) without having to ping-pong the whole dataset to and from the client. A session mechanism can be implemented at the "user" level in the application code but most of the languages used to develop web applications provide various build-ins to accomplish the task. This is the case of PHP and its famous "session" module (Session Support in phpinfo()). The $_SESSION array can be used transparently and the session has just to be started with session_start() (or even automatically started at the configuration level with session.auto_start).