Processing google calendar feed (prefixed namespace issue)

Hello PHP people. I'm trying to use simplexml to gather info from a
google calendar feed, and I've run into a problem where the google xml
has a namespace prefixed to some fields, and I don't know how to deal
with this colon. For instance:

$xml_data = simplexml_load_file($xml);

foreach ($xml_data->entry as $item) {

print "

";
print $item->title;
print "
";
print $item->gd:when;
print "