<!DOCTYPE html>
<html>
<body>

<?php
$xml=<<<XML
<book xmlns:chap="http://example.org/chapter-title">
   <title>My Book</title>
   <chapter id="1">
     <chap:title>Chapter 1</chap:title>
     <para>Donec velit. Nullam eget tellus...</para>
   </chapter>
   <chapter id="2">
     <chap:title>Chapter 2</chap:title>
     <para>Lorem ipsum dolor sit amet....</para>
   </chapter>
</book>
XML;

$sxe=new SimpleXMLElement($xml);
$sxe->registerXPathNamespace('c', 'http://example.org/chapter-title');
$result=$sxe->xpath('//c:title');
foreach ($result as $title)
   {
   echo $title . "<br>";
   }
?>

</body>
</html>