如何在PHP中使用Xpath的count函数？

<?php 

$xmlStr = file_get_contents('books.xml'); 
$xml = new SimpleXMLElement($xmlStr); 

$res = $xml->xpath("count(catalog/book)");
print_r($res); 

?>

XML：

<?xml version="1.0"?>
<catalog>
<book id="bk101">
    <author>Gambardella,Matthew</author>
    <title>XML Developer's Guide</title>
    <genre>Computer</genre>
    <price>44.95</price>
    <publish_date>2000-10-01</publish_date>
    <description>An in-depth look at creating applications 
  with XML.</description>
</book>
<book id="bk102">
    <author>Ralls,Kim</author>
    <title>Midnight Rain</title>
    <genre>Fantasy</genre>
    <price>5.95</price>
    <publish_date>2000-12-16</publish_date>
    <description>A former architect battles corporate zombies,an evil sorceress,and her own childhood to become queen 
  of the world.</description>
</book>
<book id="bk103">
    <author>Corets,Eva</author>
    <title>Maeve Ascendant</title>
    <genre>Fantasy</genre>
    <price>5.95</price>
    <publish_date>2000-11-17</publish_date>
    <description>After the collapse of a nanotechnology 
  society in England,the young survivors lay the 
  foundation for a new society.</description>
</book>
<book id="bk104">
    <author>Corets,Eva</author>
    <title>Oberon's Legacy</title>
    <genre>Fantasy</genre>
    <price>5.95</price>
    <publish_date>2001-03-10</publish_date>
    <description>In post-apocalypse England,the mysterious 
  agent known only as Oberon helps to create a new life 
  for the inhabitants of London. Sequel to Maeve 
  Ascendant.</description>
</book>
<book id="bk105">
    <author>Corets,Eva</author>
    <title>The Sundered Grail</title>
    <genre>Fantasy</genre>
    <price>5.95</price>
    <publish_date>2001-09-10</publish_date>
    <description>The two daughters of Maeve,half-sisters,battle one another for control of England. Sequel to 
  Oberon's Legacy.</description>
</book>
<book id="bk106">
    <author>Randall,Cynthia</author>
    <title>Lover Birds</title>
    <genre>Romance</genre>
    <price>4.95</price>
    <publish_date>2000-09-02</publish_date>
    <description>When Carla meets Paul at an ornithology 
  conference,tempers fly as feathers get ruffled.</description>
</book>
<book id="bk107">
    <author>Thurman,Paula</author>
    <title>Splish Splash</title>
    <genre>Romance</genre>
    <price>4.95</price>
    <publish_date>2000-11-02</publish_date>
    <description>A deep sea diver finds true love twenty 
  thousand leagues beneath the sea.</description>
</book>
<book id="bk108">
    <author>Knorr,Stefan</author>
    <title>Creepy Crawlies</title>
    <genre>Horror</genre>
    <price>4.95</price>
    <publish_date>2000-12-06</publish_date>
    <description>An anthology of horror stories about roaches,centipedes,scorpions  and other insects.</description>
</book>
<book id="bk109">
    <author>Kress,Peter</author>
    <title>Paradox Lost</title>
    <genre>Science Fiction</genre>
    <price>6.95</price>
    <publish_date>2000-11-02</publish_date>
    <description>After an inadvertant trip through a Heisenberg
  Uncertainty Device,James Salway discovers the problems 
  of being quantum.</description>
</book>
<book id="bk110">
    <author>O'Brien,Tim</author>
    <title>Microsoft .NET: The Programming Bible</title>
    <genre>Computer</genre>
    <price>36.95</price>
    <publish_date>2000-12-09</publish_date>
    <description>Microsoft's .NET initiative is explored in 
  detail in this deep programmer's reference.</description>
</book>
<book id="bk111">
    <author>O'Brien,Tim</author>
    <title>MSXML3: A Comprehensive Guide</title>
    <genre>Computer</genre>
    <price>36.95</price>
    <publish_date>2000-12-01</publish_date>
    <description>The Microsoft MSXML3 parser is covered in 
  detail,with attention to XML DOM interfaces,XSLT processing,SAX and more.</description>
</book>
<book id="bk112">
    <author>Galos,Mike</author>
    <title>Visual Studio 7: A Comprehensive Guide</title>
    <genre>Computer</genre>
    <price>49.95</price>
    <publish_date>2001-04-16</publish_date>
    <description>Microsoft Visual Studio 7 is explored in depth,looking at how Visual Basic,Visual C++,C#,and ASP+ are 
  integrated into a comprehensive development 
  environment.</description>
</book>
</catalog>

以下将在屏幕上打印出一系列书籍：

<?php 

$xmlStr = file_get_contents('books.xml'); 
$xml = new SimpleXMLElement($xmlStr); 

$res = $xml->xpath("/catalog/book");
print_r($res); 

?>

我只是无法弄清楚为什么count函数不起作用.