PHP에서 xml 노드 값을 변경하는 방법

<?xml version="1.0" encoding="utf-8"?>
<records>
<record>
<pono>5008171</pono>
<status>3</status>
<opentime>2010.06.13 14:19</opentime>
<closetime>2010.06.16 14:19</closetime>
<potype>balance</potype>
<variety/>
<margin/>
<openprice/>
<closeprice/>
<zhisun/>
<zhiying/>
<lowest/>
<highest/>
<netvalue/>
<openamount/>
<openinterest/>
<amount/>
<point/>
<positiontime>3</positiontime>
<memo>TRMM-DP(123005)-D</memo>
</record>
<record>
<pono>5011083</pono>
<status>3</status>
<opentime>2010.06.15 16:15</opentime>
<closetime>2010.06.15 16:23</closetime>
<potype>buy</potype>
<variety>eurusd</variety>
<margin/>
<openprice>1.31822</openprice>
<closeprice>1.31655</closeprice>
<zhisun>0</zhisun>
<zhiying>0</zhiying>
<lowest/>
<highest/>
<netvalue/>
<openamount/>
<openinterest/>
<amount/>
<point/>
<positiontime>00:08:00</positiontime>
<memo>aaafff</memo>
</record>
<record>
<pono>5011913</pono>
<status>3</status>
<opentime>2010.06.15 16:51</opentime>
<closetime>2010.06.15 17:19</closetime>
<potype>sell</potype>
<variety>eurusd</variety>
<margin/>
<openprice>1.31819</openprice>
<closeprice>1.31809</closeprice>
<zhisun>0</zhisun>
<zhiying>0</zhiying>
<lowest/>
<highest/>
<netvalue/>
<openamount/>
<openinterest/>
<amount/>
<point/>
<positiontime>00:28:00</positiontime>
<memo>eee</memo>
</record>
</records>

$file ="mainchart.xml";
    //创建DOMDocument的对象
    $dom=new DOMDocument('1.0');
    //载入mainchart.xml文件
    $dom->load($file);
    //获得record节点的集合
    $records = $dom->getElementsByTagName('record');
    //遍历record节点的集合
    foreach($records as $record){
        //如果record节点的pono子节点的值满足条件，就修改该record节点下memo子节点的值
        if($record->getElementsByTagName('pono')->item(0)->nodeValue == $_GET['id']){
            $record->getElementsByTagName('memo')->item(0)->nodeValue = $_GET['content'];
        }
    }
$dom->save('mainchart.xml');