xml php输出xml属性

<?php
   $xml = simplexml_load_file("books.xml");
   foreach($xml->book[0]->author->attributes() AS $a => $b) {
      echo "$a = $b <br />";
   }
?>

                                 xml 文件内容如下

<library>
   <book>
      <title>A</title>
      <author gender="female">B</author>
      <description>C</description>
   </book>
   <book>
      <title>C</title>
      <author gender="male">D</author>
      <description>E</description>
   </book>
   <book>
      <title>F</title>
      <author gender="male">G</author>
      <description>H</description>
   </book>
</library>