title: Undefined date_diff()
body: <p>I'm trying to use <code>date_diff()</code>:</p>
  
  <pre><code>$datetime1 = date_create('19.03.2010');
  $datetime2 = date_create('22.04.2010');
  $interval = date_diff($datetime1, $datetime2);
  echo $interval-&gt;format('%R%d days');
  </code></pre>
  
  <p>Its doesn't work for me, gives an error:</p>
  
  <pre><code>Call to undefined function  date_diff()
  </code></pre>
  
  <p><strong>How can I get it work?</strong></p>
  
  <p>PHP 5.2 is used.</p>
  
  <p>Thanks.</p>