title: Creating multi level classes in PHP
body: <p>I am working on a software project and have noticed when looking at other project's code, such as the Zend Framework, that they use some kind of inheriting multi-level classes.</p>
  
  <p>An example is <code>$system-&gt;sessions-&gt;closeSession()</code>. I understand that objects provide similar functionality, but I was not aware that they could be used for actual callable methods.</p>
  
  <p>How can I create a class with this technique?</p>