To implement a publish/subscribe behaviour to an object, whenever a "Subject" object changes its state, the attached "Observers" will be notified. It is used to shorten the amount of coupled objects and uses loose coupling instead.

• a message queue system is observed to show the progress of a job in a GUI

PHP already defines two interfaces that can help to implement this pattern: SplObserver and SplSubject.

You can also find this code on GitHub

User.php

.. literalinclude:: User.php
   :language: php
   :linenos:

UserObserver.php

.. literalinclude:: UserObserver.php
   :language: php
   :linenos:

Tests/ObserverTest.php

.. literalinclude:: Tests/ObserverTest.php
   :language: php
   :linenos: