<?php

$dom = new DOMDocument;
$dom->preserveWhiteSpace = FALSE;
$dom->loadXML('<root><foo><bar>baz</bar></foo></root>');
$dom->formatOutput = TRUE;
echo $dom->saveXml();

<?xml version="1.0"?>
<root>
  <foo>
    <bar>baz</bar>
  </foo>
</root>