Session Handler Class: Manage sessions stored in a MySQL database

This class can be used to manage sessions stored in a MySQL database.

It can store the identifier of a given authenticated user and a session data string in a MySQL database table.

The class can also retrieve the user identifier and the session data string into session variables.

It can also delete specific user sessions from the MySQL database table or just the sessions that have expired.

A separate table keeps track of the time when it is necessary to do session maintenance that is handled by a configurable PHP script.