关于PHP解析XML怎么通过属性查找到指定节点？

XML code:
<?xml version="1.0" encoding="gb2312"?>
<books>
<book id="1">
<author>Jack Chen</author>
<title>PHP Hacks</title>
<publisher>O'Reilly</publisher>
</book>
<book id="2">
<author>Jack Herrington</author>
<title>Podcasting Hacks</title>
<publisher>O'Reilly</publisher>
</book>
</books>

我有这样的一个XML文件
我用的是PHP DOMDocument解析
现在我想知道怎么通过book 中的id值来列出这一节点下的所有值？
也就是说，我现在想显示出
id=1的book内
author,title,publisher所有信息？

---

  <?php
  $doc = new DOMDocument();
  $doc->load( 'books.xml' );
 
  $book = $doc->getElementsById( "1" );
  $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;
 
  e