I am needing help trying to extract data out of an XML using PHP.

I have the following simple page that is passed the following type of value for URI, http://www.google.com/calendar/feeds...gclr777v7bc46s

PHP Code:
<?php
if (isset($_GET['uri'])) {
    
$uri=$_GET['uri'];
    
    
$xml=simplexml_load_file($uri);
    
    
print_r($xml);    
    
    
$title=$xml->title;
    
$content=$xml->content;
    
} else
 
header'Location: http://www.rosmini.school.nz/facebook' ) ;
?>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Event Details</title>
</head>

<body>
</body>
</html>
I have no problem getting details like title and content but what I can't get is the start and end times which are buried in the fetch XML in <gd:when endTime='2011-10-26' startTime='2011-10-25'/>

I want to be able to do something like
PHP Code:
$start=.........
$end=....... 
But when I look at the output of simplexml_load_file($uri), I can't see these values anywhere.