Can't get simplexml's xpath to return anything from flickr's photo set RSS feed

Consider the following:

$feed_url = "";
$flickr_xml = simplexml_load_file($feed_url);

$something = $flickr_xml->xpath('/feed');


If I look at the XML in the feed above, the first element its “feed”. So, that xpath expression should return something, but it doesn’t.

In fact, xpath(’/’) doesn’t return anything either. Anyone know why this is?