PHP通过DOM方式解析获得每本书的作者和书名

<?php
  $doc = new DOMDocument('1.0','utf-8');
  $doc -> load('book.xml');
  $books = $doc ->getElementsByTagName('book');
  $first_book = $books ->item(0);
  $first_title = $first_book ->getElementByTagName('title')->item(0)->nodeValue;
  $first_author = $first_book ->getElementByTagName('author')->item(0)->nodeValue;
  $second_book = $books ->item(1);
  $second_title = $second_book ->getElementsByTagName('title');
  $tar = $title ->item(0);
  $text = $tar ->nodeValue;
  $third_book = $books ->item(2);
  $third_title = $third_book ->getElementsByTagName('title')->item(0)->nodeValue;
  $third_author = $third_book ->getElementsByTagName('author')->item(0)->nodeValue;
  var_dump($third_title);
  var_dump($third_author);
?>