How do you query namespaces with PHP/XPath/DOM

I am trying to query an XML document that uses namespaces. I have had success with xpath without namespaces, but no results with namespaces. This is a basic example of what I was trying. I have condensed it slightly, so there may be small issues in my sample that may detract from my actual problem.

Sample XML:

<?xml version="1.0"?>
<sf:page>
     <sf:section>
          <sf:layout>
              <sf:p>My Content</sf:p>
          </sf:layout>
     </sf:section>
</sf:page>

Sample PHP Code:

<?php
$path = "index.xml";

$content = file_get_contents($path);

$dom = new DOMDocument($content);

$xpath = new DOMXPath($dom);
$xpath->registerNamespace('sf', "http://developer.apple.com/namespaces/sf");

$p = $xpath->query("//sf:p"开发者_开发知识库, $dom);

My result is that "p" is a "DOMNodeList Object ( )" and it's length is 0. Any help would be appreciated.

---

DOMDocument constructor does not take the contents, but version and encoding. Instead of:

$path = "index.xml";
$content = file_get_contents($path);
$dom = new DOMDocument($content);

Try this:

$path = "index.xml";
$doc = new DOMDocument();
$doc->load($path);

---

You must define namespace in your xml file:

<?xml version="1.0"?>
<root xmlns:sf="http://developer.apple.com/namespaces/sf">
    <sf:page>
         <sf:section>
              <sf:layout>
                  <sf:p>My Content</sf:p>
              </sf:layout>
         </sf:section>
    </sf:page>
</root>