Format an unformatted XML string or unformat a formatted XML string, also accepts SimpleXMLElement xml object.

format_xml.php

<?php
function format_xml($xml, $formatOutput=TRUE, $declaration=TRUE) {
    $sxe = ($xml instanceof \SimpleXMLElement) ? $xml : simplexml_load_string($xml);
    $domElement = dom_import_simplexml($sxe);
    $domDocument = $domElement->ownerDocument;
    $domDocument->preserveWhiteSpace = false;
    $domDocument->formatOutput = (bool)$formatOutput;
    $domDocument->loadXML($sxe->asXML(), LIBXML_NOBLANKS); // Fixes newlines omitted by DomNode::appendChild()
    return (bool)$declaration ? $domDocument->saveXML() : $domDocument->saveXML($domDocument->documentElement);
}

format_xmlTest.php

<?php
$xml = '<Nodes><Node id="1">Content 1</Node><Node id="2">Content 2</Node></Nodes>';
echo format_xml($xml);

/*
<?xml version="1.0"?>
<Nodes>
  <Node id="1">Content 1</Node>
  <Node id="2">Content 2</Node>
</Nodes>
*/