php+xml(php怎么改变xml中的值)

XML文件：<?xml version="1.0" encoding="gb2312"?>
 <books>
  <book>
  <author>金墉</author>
  <title>武侠小说</title>
  <publisher>科技出版社</publisher>
  <renqi>30</renqi>
  </book>
  <book>
  <author>赵晴</author>
  <title>读者</title>
  <publisher>文艺出版社</publisher>
  <renqi>58</renqi>
  </book>
   <book>
  <author>赵晴1</author>
  <title>读者1</title>
  <publisher>文艺出版社1</publisher>
  <renqi>36</renqi>
  </book>
  </books>php文件：<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>ޱĵ</title>
</head><body>
<?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;
  
   foreach ($titles as $list1)
{
$title_list = $list1->firstChild->nodeValue;
//echo "===".$title_list."<br />";
}

  $renqis =$book->getElementsByTagname("renqi");
  $renqi =$renqis->item(0)->nodeValue; foreach($book->getElementsByTagName('renqi') as $list)
{
$value = $list->firstChild->nodeValue;
//echo $value."<br />";
}
   
 // echo "$title - $author - $publisher - $renqi<br>";
 // echo  count($titles);
for($i = 0; $i < count($titles); $i++){
echo "<br>[" . $titles->item($i)->nodeValue . " ... " . $renqis->item($i)->nodeValue . "]<br>";
}
  }
  ?> <div>
        <table style="width: 606px">

<?
  foreach( $books as $book )
  { 
  $titles = $book->getElementsByTagName( "title" );
  $title = $titles->item(0)->nodeValue;
  $renqis =$book->getElementsByTagname("renqi");
  $renqi =$renqis->item(0)->nodeValue;
for($i = 0; $i < count($titles); $i++){
?>
            <tr>
                <td style="width: 105px; height: 11px">
                  <? echo $titles->item($i)->nodeValue ?></td>
                <td colspan="1" style="width: 260px; height: 11px">
                    <table style="width: <?=($renqis->item($i)->nodeValue)*5 ?>px; background-color: #ffcc66">
                        <tr>
                            <td colspan="3" rowspan="3" style="width: 6px">                            </td>
                        </tr>
                        <tr>                        </tr>
                        <tr>                        </tr>
                    </table>                </td>
                <td colspan="2" style="height: 11px">            </td>
            </tr>
           <?  
   }
   }
   ?>
   
        </table>
    
</div>
</body>
</html>怎么改变xml文件内的值呢请各位多多指教．．谢谢　．．．