The elements with colons are using namespace prefixes, and it is perfectly valid XML. When using multiple schemas together, namespaces help differentiate elements which have the same name. The way SimpleXMLElement objects in PHP handle it, you can essentially ignore the part before the colon:
$current->fid;
If you do want to find out the namespace portion of the element name, you can do:
$current->fid->getNamespaces();