The solution is explained in this nice article. You need the children()
method for accessing XML elements which contain a namespace. This code snippet is quoted from the article:
$feed = simplexml_load_file('http://www.sitepoint.com/recent.rdf');
foreach ($feed->item as $item) {
$ns_dc = $item->children('http://purl.org/dc/elements/1.1/');
echo $ns_dc->date;
}