title: Php Class 'DateInterval' not found
body: <p>I am using php version 5.4.24 and I can't use the class DateInterval.<br>
  I get this error: <strong>PHP Fatal error:  Class 'DateInterval' not found</strong>  </p>
  
  <pre><code>$interval       = new DateInterval('P91D');
  </code></pre>
  
  <p>What should I do in order to make this code work?</p>