Decouple an abstraction from its implementation so that the two can vary independently.

You can also find this code on GitHub

Formatter.php

.. literalinclude:: Formatter.php
   :language: php
   :linenos:

PlainTextFormatter.php

.. literalinclude:: PlainTextFormatter.php
   :language: php
   :linenos:

HtmlFormatter.php

.. literalinclude:: HtmlFormatter.php
   :language: php
   :linenos:

Service.php

.. literalinclude:: Service.php
   :language: php
   :linenos:

HelloWorldService.php

.. literalinclude:: HelloWorldService.php
   :language: php
   :linenos:

PingService.php

.. literalinclude:: PingService.php
   :language: php
   :linenos:

Tests/BridgeTest.php

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