You may need to save all or part of a DOMDocument as an XHTML-friendly string, something compliant with both XML and HTML 4. Here's the DOMDocument class extended with a saveXHTML method:

<?php

/**
 * XHTML Document
 *
 * Represents an entire XHTML DOM document; serves as the root of the document tree.
 */
class XHTMLDocument extends DOMDocument {

  /**
   * These tags must always self-terminate. Anything else must never self-terminate.
   * 
   * @var array
   */
  public $selfTerminate = array(
      'area','base','basefont','br','col','frame','hr','img','input','link','meta','param'
  );
  
  /**
   * saveXHTML
   *
   * Dumps the internal XML tree back into an XHTML-friendly string.
   *
   * @param DOMNode $node
   *         Use this parameter to output only a specific node rather than the entire document.
   */
  public function saveXHTML(DOMNode $node=null) {
    
    if (!$node) $node = $this->firstChild;
    
    $doc = new DOMDocument('1.0');
    $clone = $doc->importNode($node->cloneNode(false), true);
    $term = in_array(strtolower($clone->nodeName), $this->selfTerminate);
    $inner='';
    
    if (!$term) {
      $clone->appendChild(new DOMText(''));
      if ($node->childNodes) foreach ($node->childNodes as $child) {
        $inner .= $this->saveXHTML($child);
      }
    }
    
    $doc->appendChild($clone);
    $out = $doc->saveXML($clone);
    
    return $term ? substr($out, 0, -2) . ' />' : str_replace('><', ">$inner<", $out);

  }

}

?>

This hasn't been benchmarked, but is probably significantly slower than saveXML or saveHTML and should be used sparingly.