You're dealing with namespaces, and this Sitepoint article looks like a good long explanation. Or for a more concise version, look here in the PHP SimpleXML
docs.
From the docs:
<?php
$xml = '<example xmlns:foo="my.foo.urn">
<foo:a>Apple</foo:a>
<foo:b>Banana</foo:b>
<c>Cherry</c>
</example>';
$sxe = new SimpleXMLElement($xml);
$kids = $sxe->children('foo');
var_dump(count($kids));
$kids = $sxe->children('foo', TRUE);
var_dump(count($kids));
$kids = $sxe->children('my.foo.urn');
var_dump(count($kids));
$kids = $sxe->children('my.foo.urn', TRUE);
var_dump(count($kids));
$kids = $sxe->children();
var_dump(count($kids));
?>
Outputs:
int(0)
int(2)
int(2)
int(0)
int(1)