Gumbo's solution does the trick. You can do work with simpleXml above and then add this at the end to echo and/or save it with formatting.
Code below echos it and saves it to a file (see comments in code and remove whatever you don't want):
//Format XML to save indented tree rather than one line
$dom = new DOMDocument('1.0');
$dom->preserveWhiteSpace = false;
$dom->formatOutput = true;
$dom->loadXML($simpleXml->asXML());
//Echo XML - remove this and following line if echo not desired
echo $dom->saveXML();
//Save XML to file - remove this and following line if save not desired
$dom->save('fileName.xml');