title: Cakephp Class 'DOMDocument' not found
body: <p>I tried to implement rss feed following this simple explanation from cake book <a href="http://book.cakephp.org/2.0/en/core-libraries/helpers/rss.html">http://book.cakephp.org/2.0/en/core-libraries/helpers/rss.html</a>
  I got this error 
     "Class 'DOMDocument' not found in lib/Cake/Utility/Xml.php on line 99"</p>
  
  <p>Any clue?</p>