title: DateTime->format("U") doesn't return a value with timezone info
body: <p>In PHP 5.2, I'm using the following code to get a timestamp from a DateTime object.</p>
  
  <pre><code>$dateTime = new DateTime("now", new DateTimeZone("America/Los_Angeles") );
  
  echo $dateTime-&gt;format("U");
  </code></pre>
  
  <p>the problem is that <strong>format("U")</strong> simply returns server timestamp, which is UTC.</p>
  
  <p>How do I make it to return a timestamp from Pacific Time Zone (Los Angeles) ?</p>