$doc = new DOMDocument();
$doc->load('test.xml');
$user = $doc->getElementsByTagName('uname');
echo $user->item(0)->nodeValue;
$old = $doc->createElement($user->item(0)->nodeName, $user->item(0)->nodeValue);
$new = $doc->createElement('uname', 'not a test');

$new = $doc->replaceChild($new, $old);
echo $doc->saveXML();