<?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>";
}
?>