dom_fragment.php

<?php

  $xml = <<<EOD
<?xml version="1.0" encoding="UTF-8"?>
<cmrl xmlns:dotgo="http://dotgo.com/cmrl/1.0" account="jddbb-ztc2l-f1tb7-2c8st">
<match pattern="">
    <message><content>DOTGO.COM<br />
      Welcome to DOTGO!<br />
      <br />
      Reply:<br />
      <a query="dotgo about" /> What is DOTGO?<br />
      <a query="dotgo blog" /> The DOTGO blog<br />
      <a query="dotgo publishing" /> Publishing with DOTGO<br />
      <a query="dotgo contact" /> Contact us<br />
      <br />
      Ex:<br />
      3
    </content></message>
  </match>
</cmrl>
EOD;

  $doc = new DOMDocument();
  $doc->loadXML($xml);
  $matches = $doc->getElementsByTagName("match");
  $match = $matches->item(0);
  $f = $doc->createDocumentFragment();
  $arg = $_POST['content'];
  $arg = str_replace(array('<![CDATA[', ']]>'), '', $arg);
  $arg = '<content>'.stripslashes($arg).'</content>';
  
  $f->appendXML($arg);
  $match->replaceChild($f, $match->firstChild);
  echo $doc->saveXML();
}