ajout d'un element dans un document XML existant

bonjour
voici mon code:

$container_url= "c://train.dbxml";

$docname = "Tunis";
echo $docname;

$mgr = new XmlManager();

$qc = $mgr->createQueryContext();
$uc = $mgr->createUpdateContext();
$mod = $mgr->createModify();
if ( !($mgr->existsContainer($container_url))) {
$container = $mgr->createContainer($container_url);
}
else{
$container = $mgr->openContainer($container_url);
}

try {
$progs = $container->getDocument($docname);
} catch (Exception $e) {
$progs = $mgr->createDocument();
$progs->setName($docname);
$container->putDocument($progs, $uc);
}
$select = $mgr->prepare("/depart", $qc);
$i=$mod->Element;
$mod->addAppendStep($select,$i,"sfax",NULL);
$doc = $container->getDocument($docname);

$s = $doc->getContentAsString();
echo "$s ";

$retDoc2 = $container->getDocument("Tunis");
$mssg = $retDoc2->getName() .":\n";
$mssg .= $retDoc2->getContentAsString();
echo $mssg;
unset($doc);
unset($progs);
unset($container);

mon problème est que rien n'est ajouté dans la base dbxml
j'aimrais bien avoir une solution
merci