<?php

$data = <<<XML
<sample>
  <content>Inhalt Eins</content>
  <content>Inhalt <b>Zwei</b></content>
</sample>
XML;

$dom = new DOMDocument();
$dom->loadXml($data);
$xpath = new DOMXpath($dom);

// let's look for all content elements
foreach ($xpath->evaluate('//content') as $contentNode) {
  // save the fragments into an string
  $fragment = '';
  foreach ($contentNode->childNodes as $fragmentNode) {
    $fragment = $dom->saveXml($fragmentNode);
  }
  // and create an cdata from that string
  $cdataNode = $dom->createCDATASection($fragment);
  // remove all children of the content node
  while ($contentNode->firstChild) {
    $contentNode->removeChild($contentNode->firstChild);
  }
  // and add the cdata as the only new child node
  $contentNode->appendChild($cdataNode);
}

header('Content-type: text/xml');
echo $dom->saveXml();