The Visitor Pattern lets you outsource operations on objects to other objects. The main reason to do this is to keep a separation of concerns. But classes have to define a contract to allow visitors (the Role::accept method in the example).

The contract is an abstract class but you can have also a clean interface. In that case, each Visitor has to choose itself which method to invoke on the visitor.

You can also find this code on GitHub

RoleVisitorInterface.php

.. literalinclude:: RoleVisitorInterface.php
   :language: php
   :linenos:

RoleVisitor.php

.. literalinclude:: RoleVisitor.php
   :language: php
   :linenos:

Role.php

.. literalinclude:: Role.php
   :language: php
   :linenos:

User.php

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

Group.php

.. literalinclude:: Group.php
   :language: php
   :linenos:

Tests/VisitorTest.php

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