This class can authenticate users with records stored in MySQL.
It can connect to a MySQL database using the PDO extension and executes a SQL query to verify whether a given user and password are from a valid users and the password is correct.
If found, the class initializes a few session variables with the user account and role information.
The class can also check if the session variables are set to determine if the user is logged. In that case, the class can logout the user by clearing the session variables.