如何通过PHP基于dom实现读取图书xml格式数据功能？

<?php
 $doc = new DOMDocument();
 $doc->load( 'books.xml' );
 $books = $doc->getElementsByTagName( "book" );
 foreach( $books as $book )
 {
 $authors = $book->getElementsByTagName( "author" );
 $author = $authors->item(0)->nodeValue;
 $publishers = $book->getElementsByTagName( "publisher" );
 $publisher = $publishers->item(0)->nodeValue;
 $titles = $book->getElementsByTagName( "title" );
 $title = $titles->item(0)->nodeValue;
 echo "$title - $author - $publisher\n";
 }
?>

<?xml version="1.0"?>
<books>
 <book>
 <author>Jack Herrington</author>
 <title>PHP Hacks</title>
 <publisher>O'Reilly</publisher>
 </book>
 <book>
 <author>Jack Herrington</author>
 <title>Podcasting Hacks</title>
 <publisher>O'Reilly</publisher>
 </book>
</books>

PHP Hacks - Jack Herrington - O'Reilly
Podcasting Hacks - Jack Herrington - O'Reilly