Accessing a specific node which is also used in other areas in the file

If I have the following data in my XML file;

<?xml version="1.0" encoding="UTF-8"?>
<Document xmlns="urn:iso:std:iso:20022:tech:xsd:pain.008.001.02" xmlns:xsi="http://ift.tt/ra1lAU">
<CstmrDrctDbtInitn>
<PmtInf>
      <PmtInfId>5n7gfUaPGK</PmtInfId>
      <PmtMtd>DD</PmtMtd>
      <NbOfTxs>1</NbOfTxs>
      <CtrlSum>200.2</CtrlSum>
      <PmtTpInf>
        <SvcLvl>
          <Cd>SEPA</Cd>
        </SvcLvl>
        <LclInstrm>
          <Cd>CORE</Cd>
        </LclInstrm>
        <SeqTp>RCUR</SeqTp>
      </PmtTpInf>
      <DrctDbtTxInf>
      <PmtId>
          <EndToEndId>nmu5AOhE7G</EndToEndId>
      </PmtId>
      </DrctDbtTxInf>
 </PmtInf>
 <PmtInf>
      <PmtInfId>5jAcoNoId3</PmtInfId>
      <PmtMtd>DD</PmtMtd>
      <NbOfTxs>3</NbOfTxs>
      <CtrlSum>100.5</CtrlSum>
      <PmtTpInf>
        <SvcLvl>
          <Cd>SEPA</Cd>
        </SvcLvl>
        <LclInstrm>
          <Cd>CORE</Cd>
        </LclInstrm>
        <SeqTp>FRST</SeqTp>
      </PmtTpInf>
      <DrctDbtTxInf>
          <PmtId>
              <EndToEndId>nmu5AbdfG</EndToEndId>
          </PmtId>
      </DrctDbtTxInf>
      <DrctDbtTxInf>
          <PmtId>
              <EndToEndId>nmu5A3r5jgG</EndToEndId>
          </PmtId>
      </DrctDbtTxInf>
</PmtInf>
</CstmrDrctDbtInitn>
</Document>

How would I access <NbOfTxs> in the second <PmtInf> block (where the value is 3) instead of <NbOfTxs> in the first <PmtInf> block (where the value is 1)?

If I just used the following line of code;

$FRSTTransaction = $xml->getElementsByTagName('NbOfTxs')->nodeValue;

It doesn't know which <NbOfTxs> I am attempting to access.

The only difference between each payment block is the <SeqTp>. There will be 4 Payment Blocks in total.

I am trying to count the number of <DrctDbtTxInf>blocks in each Payment Block and then put this value into <NbOfTxs>.

      <PmtId>
          <EndToEndId>nmu5AOhE7G</EndToEndId>
      </PmtId>
      </DrctDbtTxInf>

The code I tried is as follows;

$filename = date('Y-W').'.xml'; //2014-26.xml
$xml = new DOMDocument;
$xml->load($filename);
$NumberTransactions = 0;
$RCURTransaction = $xml->getElementsByTagName('DrctDbtTxInf');
$NodeValue = $xml->getElementsByTagName('NbOfTxs')->nodeValue;
foreach ($RCURTransaction as $Transaction) {
$NodeValue = $NodeValue + 1;
}

$Document = simplexml_load_file($filename);
$Document->CstmrDrctDbtInitn->PmtInf->NbOfTxs = $NodeValue; 
$Document->asXML($filename);

I receive no errors, it just doesnt seem to access the node value.