There’s a variety of solutions in the comments on the PHP manual page for SimpleXMLElement. Not very efficient, but certainly terse, is a solution by Anonymous
$dom = dom_import_simplexml($simpleXml)->ownerDocument;
$dom->formatOutput = true;
echo $dom->saveXML();
The PHP manual page comments are often good sources for common needs, as long as you filter out the patently wrong stuff first.