XML在PHP中获得嵌套节点

在这个读取XML文件的php脚本中，用于访问嵌套节点的内部循环不返回任何数据。

<?php
// load SimpleXML
$nodes = new SimpleXMLElement('communities.xml', null, true);
echo <<<EOF
<table>
        <tr>
                <th>ID</th>
                <th>Name</th>
                <th>Top</th>
                <th>Left</th>
                <th>Width</th>
                <th>Height</th>
                <th>Urls</th>
        </tr>
EOF;
foreach($nodes as $node) // loop through 
{
       echo "<tr>";
       echo "        <td>".$node['ID']."</td>";
       echo "         <td>".$node->NAME."</td>";
       echo "         <td>".$node->TOP."</td>";
       echo "         <td>".$node->LEFT."</td>";
       echo "         <td>".$node->WIDTH."</td>";
       echo "         <td>".$node->HEIGHT."</td>";
       echo "         <td>";
                $urls = $node->URLS;
                /* $urls = $xml->xpath('/COMMUNITIES/COMMUNITY/URLS/URL'); */
                foreach($urls as $url)
                {
                    echo ' '.$url->NAME;
                }
       echo '        </td>';
        echo '</tr>';
}
echo '</table>';
?>

<?xml version="1.0" encoding="ISO-8859-1" ?> 
<COMMUNITIES>
<COMMUNITY ID="c001">
  <NAME>Town Services</NAME> 
  <TOP>50</TOP> 
  <LEFT>50</LEFT> 
  <WIDTH>200</WIDTH> 
  <HEIGHT>300</HEIGHT> 
  <URLS>
      <URL ID="U001">
          <NAME>Google.com</NAME>
          <URL>http://www.google.com</URL>
      </URL>
      <URL ID="U002">
          <NAME>Bing.com</NAME>
          <URL>http://www.bing.com</URL>
      </URL>
      <URL ID="U003">
          <NAME>Yahoo.com</NAME>
          <URL>http://www.yahoo.com</URL>
      </URL>
      <URL ID="U004">
          <NAME>Aol.com</NAME>
          <URL>http://www.aol.com</URL>
      </URL>
  </URLS> 
  </COMMUNITY>
<COMMUNITY ID="c002">
  <NAME>Local Stores</NAME> 
  <TOP>50</TOP> 
  <LEFT>260</LEFT> 
  <WIDTH>200</WIDTH> 
  <HEIGHT>150</HEIGHT> 
  <URLS>
      <URL ID="U001">
          <NAME>Walgreens</NAME>
          <URL>http://www.walgreens.com</URL>
      </URL>
      <URL ID="U002">
          <NAME>Bing.com</NAME>
          <URL>http://www.bing.com</URL>
      </URL>
      <URL ID="U003">
          <NAME>Yahoo.com</NAME>
          <URL>http://www.yahoo.com</URL>
      </URL>
  </URLS> 
  </COMMUNITY>
<COMMUNITY ID="c003">
  <NAME>Attractions</NAME> 
  <TOP>50</TOP> 
  <LEFT>470</LEFT> 
  <WIDTH>200</WIDTH> 
  <HEIGHT>300</HEIGHT> 
  <URLS>
      <URL ID="U001">
          <NAME>Museum</NAME>
          <URL>http://www.mfa.org</URL>
      </URL>
      <URL ID="U002">
          <NAME>Park</NAME>
          <URL>http://www.bing.com</URL>
      </URL>
  </URLS> 
  </COMMUNITY>
<COMMUNITY ID="c004">
  <NAME>Online Stores</NAME> 
  <TOP>370</TOP> 
  <LEFT>50</LEFT> 
  <WIDTH>200</WIDTH> 
  <HEIGHT>150</HEIGHT> 
  <URLS>
      <URL ID="U001">
          <NAME>Amazon.com</NAME>
          <URL>http://www.amazon.com</URL>
      </URL>
      <URL ID="U002">
          <NAME>Target.com</NAME>
          <URL>http://www.target.com</URL>
      </URL>
  </URLS> 
  </COMMUNITY>
<COMMUNITY ID="c005">
  <NAME>Online Forums</NAME> 
  <TOP>370</TOP> 
  <LEFT>300</LEFT> 
  <WIDTH>200</WIDTH> 
  <HEIGHT>200</HEIGHT> 
  <URLS>
      <URL ID="U001">
          <NAME>Technet</NAME>
          <URL>http://www.Microsoft.com</URL>
      </URL>
      <URL ID="U002">
          <NAME>MSDN</NAME>
          <URL>http://www.Microsoft.com</URL>
      </URL>
  </URLS> 
  </COMMUNITY>
<COMMUNITY ID="c006">
  <NAME>Travel</NAME> 
  <TOP>370</TOP> 
  <LEFT>480</LEFT> 
  <WIDTH>200</WIDTH> 
  <HEIGHT>200</HEIGHT> 
  <URLS>
      <URL ID="U001">
          <NAME>Southwest</NAME>
          <URL>http://www.mfa.org</URL>
      </URL>
      <URL ID="U002">
          <NAME>Northwest</NAME>
          <URL>http://www.bing.com</URL>
      </URL>
  </URLS> 
  </COMMUNITY>
</COMMUNITIES>

我也尝试使用xpath访问内部节点，但返回错误。

$urls = $xml->xpath('/COMMUNITIES/COMMUNITY/URLS/URL');

访问内部节点的正确方式是什么?谢谢。