<?php

$sxe = simplexml_load_string('<books><book><title>blah</title></book></books>');

if ($sxe === false) {
    echo 'Erreur lors de l\'analyse du document';
    exit;
}

$dom_sxe = dom_import_simplexml($sxe);
if (!$dom_sxe) {
    echo 'Erreur lors de la convertion du XML';
    exit;
}

$dom = new DOMDocument('1.0');
$dom_sxe = $dom->importNode($dom_sxe, true);
$dom_sxe = $dom->appendChild($dom_sxe);

echo $dom->saveXML();

?>