Simple PHP User Authentication: Authenticate users with records in a MySQL table

This class can authenticate users with records in a MySQL table.

It can connect to a MySQL database using the MySQLi extension and validate a user with a given password.

If the user password is correct, the class starts a PHP session and updates the user database records with his logged user session identifier.

The class can also check later if the user is already logged and log out the user destroying his user session.