title: Strange warning when trying to extend class
body: <p>I started getting strange warnings, after extending a class that extends another
  <code>Event_Feed_Item &gt; My_Feed_Item &gt; SimplePie_Item</code></p>
  
  <pre><code>- Cannot extend from interface 
   'SimplePie_Item'
  - Implementors of methods in 
   'SimplePie_Item'
  </code></pre>
  
  <p>However on runtime it works fine. Also SimplePie_Item is not an interface, it's a simple Class..
  Just need to know if I'm running into trouble, or if i can just ignore this warning</p>
  
  <p><strong>EDIT</strong> I'm extending <a href="https://github.com/simplepie/simplepie/blob/master/SimplePie/Item.php" rel="nofollow">this class</a><br>
  Also this is the skeleton from my classes</p>
  
  <pre><code>class My_Feed_Item extends SimplePie_Item // error is triggered here
  {
      protected $_source_id = null;
  
      public function set_source ($source_id)
      {
      }
      public function get_image ()
      {
      }
      public function get_venue_id ()
      {
      }
      public function get_venue_nm ()
      {
      }
      public function cleanString($str)
      {
      }
  }
  
  class Event_Feed_Item extends My_Feed_Item
  {
      public function get_event_nm ()
      {
  
      }
      public function get_event_start_dt ()
      {
  
      }
      public function get_event_end_dt ()
      {
      }
  
  }
  </code></pre>