(PHP 4)
xpath_new_context — Creates new xpath context
Creates a new xpath context.
WARNING FOR PHP5 USERS! (if you are having xpath problems)
There seem to be several versions of the DOM library/extension arround, it would appear that the one in this page is for PHP4 only, while this is the one in PHP5:
http://us2.php.net/manual/en/ref.dom.php
See the DOMXPath class for xpath functions.
Should that not be placed as:
DomDocument->xpath_new_context as it seems to be a method used from a DomDocument object.
Split easily XPath result into associative array:
function xpath_parser($xml_file, $xpath_query) {
    $xml_file = realpath($xml_file);
    if (file_exists($xml_file)) {
        $doc = domxml_open_file($xml_file);
        $ctx = $doc->xpath_new_context();            
        if ($xpathObj = @$ctx->xpath_eval($xpath_query)) {
            $return = array();
            foreach($xpathObj->nodeset as $risultato) {
                $tagName = $risultato->tagname();
                $content = $risultato->get_content();
                if (array_key_exists($tagName, $return)) {
                    $return[$tagName][] = $content;
                }else {
                    if (count($doc->get_elements_by_tagname($tagName)) > 1) {
                        $return[$tagName] = array($content);
                    }else {
                        $return[$tagName] = $content;
                    }
                }
            }
        }else {  return("<b>Error :</b> Wrong XPath query");  }
    }else {  return("<b>Errore :</b> File not exist or wrong filepath!");  }
    return $return;
}
usage:
$xml_file = "./listaCD.xml";
$query = "/listacd/*";
echo "<pre>";
print_r(xpath_parser($xml_file, $query));
echo "</pre>";
I found a nice function in XML_sql2xml. With it, you can get, easily,  the result of a xpath expression.
function getXpathValues(&$dom, $expr) {
    $xpth = $dom->xpath_new_context();
        $xnode = xpath_eval($xpth,$expr);
        if (isset ($xnode->nodeset[0])) {
            $firstnode = $xnode->nodeset[0];
            $children = $firstnode->children();
            $value = $children[0]->content;
        return $value;
    }
        else return Null;
}
Usage :
$dom = domxml_open_file('pear.rss');
echo $this->getXpathValues($dom, "//channel/title");