PHP Updating XML Node Value

I have the following section in my XML file;

<CstmrDrctDbtInitn>
    <GrpHdr>
      <MsgID>FKichbSczD</MsgID>
      <CreDtTm>2014-07-08T12:10:00</CreDtTm>
      <NbOfTxs>1</NbOfTxs>
      <CtrlSum>400.4</CtrlSum>
      <InitgPty>
        <Id>
          <PrvtId>
            <Othr>
              <Id>IE40ZZZ303504</Id>
            </Othr>
          </PrvtId>
        </Id>
      </InitgPty>
    </GrpHdr>
</CstmrDrctDbtInitn>

How would I go about changing the value of <NbOfTxs>?

I tried the following code attempt;

 function CountTransactions($xml_file) {
    $filename = date('Y-W').'.xml'; //2014-26.xml
    $xml = new DOMDocument;
    $xml->load($filename);
    $xpath = new DOMXPath($xml);

    $Transaction = $xml->getElementsByTagName('DrctDbtTxInf');
    $NumberTransactions = 0;
    foreach ($Transaction as $Payment) {
    $NumberTransactions = $NumberTransactions + 1;
    }

    // add more elements to xml file
    $CustomerDDInfo = $xml->getElementsByTagName('CstmrDrctDbtInitn')->item(0);
    // add this element to the root
    $xml->documentElement->appendChild($CustomerDDInfo);
    // add more elements to xml file
    $GroupHeader = $xml->getElementsByTagName('GrpHdr')->item(0);
    // add this element to the root
    $CustomerDDInfo->appendChild($GroupHeader);

    $query = $xpath->query('NbOfTxs',$GroupHeader);
    $query->item(0)->nodeValue = $NumberTransactions; 
    $xml->save($xml_file);
    }

I am trying to count the number of transactions in the XML file each time it is updated. A single transaction is identified between the <DrctDbtTxInf> and </DrctDbtTxInf> tags.