//No need to initiate, import and append on example#1
(...)
$dom_sxe = dom_import_simplexml($sxe);
if (!$dom_sxe) {
echo 'Erreur lors de la conversion du XML';
exit;
}
//$dom = new DOMDocument('1.0');
//$dom_sxe = $dom->importNode($dom_sxe, true);
//$dom_sxe = $dom->appendChild($dom_sxe);
//use ownerDocument propertie
echo $dom->ownerDocument->saveXML();
?>