title: Convert DateInterval object to seconds in php
body: <pre><code>$datetime1 = date_create('2009-10-11');
  $datetime2 = date_create('2009-10-13');
  $interval = date_diff($datetime1, $datetime2);
  </code></pre>
  
  <p>How do i convert the above <code>$interval</code> to seconds in php</p>