@chris: You're wrong. XML relies on URIs and you often see ns0,ns1,ns2 in autogenerated XML. Using URIs turns out to be a totally retarded decision if everybody wants to use nicknames anyway. But that's the way it is.
@lorna: Instead of getNameSpaces() I'd personally set up a predefined array. You can define your own qualifiers then and the XMLNS URIs don't change anyway.
$xmlns = array(""=>"http://www.w3.org/2005/Atom", "os"=>"http://a9.com/-/spec/opensearch/1.1/");