How to extract html comments and all html contained by node?

Comment nodes should be easy to find in XPath with the comment() test, analogous to the text() test:

$comments = $xpath->query('//comment()'); // or another path, as you prefer

They are standard nodes: here is the manual entry for the DOMComment class.

To your other question, it's a bit trickier. The simplest way is to use saveXML() with its optional $node argument:

$html = $dom->saveXML($el);  // $el should be the element you want to get 
                             // the HTML for

For the HTML comments a fast method is:

 function getComments ($html) {

     $rcomments = array();
     $comments = array();

     if (preg_match_all('#<\!--(.*?)-->#is', $html, $rcomments)) {

         foreach ($rcomments as $c) {
             $comments[] = $c[1];
         }

         return $comments;

     } else {
         // No comments matchs
         return null;
     }

 }