asXML表现不正常

I may be missing something really obvious but this

<?php
$string = <<<XML
<a>
 <b>
  <c>text</c>
  <c>stuff</c>
 </b>
 <d>
  <c>code</c>
 </d>
</a>
XML;

$xml = new SimpleXMLElement($string);

echo $xml->asXML();

is supposed to output the below according to php.net.

<?xml version="1.0"?>
<a>
 <b>
  <c>text</c>
  <c>stuff</c>
 </b>
 <d>
  <c>code</c>
 </d>
</a>

Not the case, it is, in fact, outputting the below (for me at least). Does anyone have any ideas?

text stuff code