Session functions provide a unique identifier to a user, which can then be used to store and acquire information linked to that ID. When a visitor accesses a session-enabled page, either a new identifier is allocated, or the user is reassociated with one that was already established in a previous visit. Any variables that have been associated with the session become available to your code through the $_SESSION superglobal.
Session state is usually stored in a temporary file, although you can implement database storage using a function called session_set_save_handler(). The use of session_set_save_handler() and a discussion about other advanced session functionality are beyond the scope of this book, but you can find more information in the PHP Manual section for sessions for all items not discussed here.