如何通过PHP编辑和删除XML中的特定节点

我浪费了很多时间试图通过php编辑和删除xml文件中的特定节点。现在我成功地删除了那个节点，但当我尝试编辑一个节点时，它只会更新最后一个节点。

我的xml文件如下所示：

<?xml version="1.0"?>
  <reviews>
   <product name="NokiaN9White">
      <user name="testuser" id="02232012062451">
         <username>test@test.com</username>
         <date_post>23-02-2012</date_post>
         <date_update>23-02-2012</date_update>
         <review>Hello dear.</review>
      </user>
      <user name="testuser" id="02232012062521">
         <username>test@test.com</username>
         <date_post>23-02-2012</date_post>
         <date_update>23-02-2012</date_update>
         <review>Mrr</review>
      </user>
      <user name="testuser" id="02232012062523">
         <username>test@test.com</username>
         <date_post>23-02-2012</date_post>
         <date_update>23-02-2012</date_update>
         <review>adfasdf</review>
      </user>
   </product>
</reviews>

我编辑这些数据的代码是：

$xmlDoc = new DOMDocument(); 
$xmlDoc->load(dirname(__FILE__)."/reviews.xml");
$products = $xmlDoc->getElementsByTagName( "product" );
    foreach( $products as $product )
    {
        $prdoductN = $product->getAttribute( "name" );
        if( $prdoductN == $product_name ){
            $users = $product->getElementsByTagName( "user" );
            $userR = get_userName($_SESSION['userName']);
            foreach( $users as $user ){
                $userName = $user->getAttribute( "name" );
                $review_id = $user->getAttribute( "id" );
                if( $userName == $userR && $review_id == $r_id ){
                    $current_date = date('d-m-Y');
                    $eventN = $user->getElementsByTagName("date_update");
                    $eventN->item(0)->nodeValue = $current_date;
                    $eventC = $user->getElementsByTagName("review");
                    $eventC->item(0)->nodeValue = $comments;
                }
            }
            $xmlDoc->formatOutput = true;
            //$xmlDoc->saveXML(); // This will return the XML as a string
            $xmlDoc->save(dirname(__FILE__)."/reviews.xml");
        }
    }