To implement a loosely coupled architecture in order to get better testable, maintainable and extendable code.

DatabaseConfiguration gets injected and DatabaseConnection will get all that it needs from $config. Without DI, the configuration would be created directly in DatabaseConnection, which is not very good for testing and extending it.

• The Doctrine2 ORM uses dependency injection e.g. for configuration that is injected into a Connection object. For testing purposes, one can easily create a mock object of the configuration and inject that into the Connection object
• Symfony and Zend Framework 2 already have containers for DI that create objects via a configuration array and inject them where needed (i.e. in Controllers)

You can also find these code on GitHub

DatabaseConfiguration.php

.. literalinclude:: DatabaseConfiguration.php
   :language: php
   :linenos:

DatabaseConnection.php

.. literalinclude:: DatabaseConnection.php
   :language: php
   :linenos:

Tests/DependencyInjectionTest.php

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