title: Get all defined classes of a parent class in php
body: <p>I need to determine, after all files have been included, which classes extend a parent class, so:</p>
  
  <pre><code>class foo{
  }
  class boo extends foo{
  }
  class bar extends foo{
  }
  </code></pre>
  
  <p>and I'd like to be able to grab an array like:</p>
  
  <pre><code>array('boo','bar');
  </code></pre>