PHP Notification System: Store and retrieve user notifications in MySQL

This class can store and retrieve user notifications in MySQL.

It can store in a MySQL database table user status notifications.

The status include the subject, the actor user involved in the notification and the status indicating if the user has seen the notification already or not.

The class can retrieve the last unseen notifications for a given user.

Once the user has seen the notification, the class can change its status to seen.