This pattern provides an easy way to decouple many components working together. It is a good alternative to Observer IF you have a "central intelligence", like a controller (but not in the sense of the MVC).
All components (called Colleague) are only coupled to the MediatorInterface and it is a good thing because in OOP, one good friend is better than many. This is the key-feature of this pattern.
You can also find this code on GitHub
.. literalinclude:: MediatorInterface.php :language: php :linenos:
.. literalinclude:: Mediator.php :language: php :linenos:
.. literalinclude:: Colleague.php :language: php :linenos:
.. literalinclude:: Subsystem/Client.php :language: php :linenos:
.. literalinclude:: Subsystem/Database.php :language: php :linenos:
.. literalinclude:: Subsystem/Server.php :language: php :linenos:
.. literalinclude:: Tests/MediatorTest.php :language: php :linenos: