PHP: XML C14n for SEPA container file hash

Question

Similarly to this question, I'm trying to calculate a document hash value for a SEPA container file. This is the container my code currently generates:

<?xml version="1.0" encoding="UTF-8"?>
<conxml xmlns="urn:conxml:xsd:container.nnn.003.02" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:conxml:xsd:container.nnn.003.02 container.nnn.003.02.xsd">
  <ContainerId>
    <SenderId>TEST</SenderId>
    <IdType>EBIC</IdType>
    <TimeStamp>090237000</TimeStamp>
  </ContainerId>
  <CreDtTm>2014-02-14T09:32:37+01:00</CreDtTm>
  <MsgPain008>
    <HashValue>942AB2F57DBAF6302EDC526472098DF38C540EB75E1913DAB0DF416D168C3253</HashValue>
    <HashAlgorithm>SHA256</HashAlgorithm>
    <Document xmlns="urn:iso:std:iso:20022:tech:xsd:pain.008.003.02">
      <CstmrDrctDbtInitn>
        <GrpHdr>
          <MsgId>testmsg-1</MsgId>
          <CreDtTm>2014-02-14T09:32:37+01:00</CreDtTm>
          <NbOfTxs>1</NbOfTxs>
          <CtrlSum>0.01</CtrlSum>
          <InitgPty>
            <Nm>TestCo</Nm>
          </InitgPty>
        </GrpHdr>
        <PmtInf>
          <PmtInfId>testmsg-1-pmt-1</PmtInfId>
          <PmtMtd>DD</PmtMtd>
          <BtchBookg>true</BtchBookg>
          <NbOfTxs>1</NbOfTxs>
          <CtrlSum>0.01</CtrlSum>
          <PmtTpInf>
            <SvcLvl>
              <Cd>SEPA</Cd>
            </SvcLvl>
            <LclInstrm>
              <Cd>CORE</Cd>
            </LclInstrm>
            <SeqTp>OOFF</SeqTp>
          </PmtTpInf>
          <ReqdColltnDt>2014-02-14+01:00</ReqdColltnDt>
          <Cdtr>
            <Nm>TestCo</Nm>
          </Cdtr>
          <CdtrAcct>
            <Id>
              <IBAN>DE54100000000000012345</IBAN>
            </Id>
          </CdtrAcct>
          <CdtrAgt>
            <FinInstnId>
              <BIC>MARKDEF1100</BIC>
            </FinInstnId>
          </CdtrAgt>
          <CdtrSchmeId>
            <Id>
              <PrvtId>
                <Othr>
                  <Id>DE46ZZZ00000012345</Id>
                  <SchmeNm>
                    <Prtry>SEPA</Prtry>
                  </SchmeNm>
                </Othr>
              </PrvtId>
            </Id>
          </CdtrSchmeId>
          <DrctDbtTxInf>
            <PmtId>
              <EndToEndId>testmsg-1-2</EndToEndId>
            </PmtId>
            <InstdAmt Ccy="EUR">0.01</InstdAmt>
            <DrctDbtTx>
              <MndtRltdInf>
                <MndtId>Test-1</MndtId>
                <DtOfSgntr>2014-02-14+01:00</DtOfSgntr>
              </MndtRltdInf>
            </DrctDbtTx>
            <DbtrAgt>
              <FinInstnId>
                <BIC>MARKDEF1200</BIC>
              </FinInstnId>
            </DbtrAgt>
            <Dbtr>
              <Nm>Other Test</Nm>
            </Dbtr>
            <DbtrAcct>
              <Id>
                <IBAN>DE90200000000000098765</IBAN>
              </Id>
            </DbtrAcct>
            <RmtInf>
              <Ustrd>Test</Ustrd>
            </RmtInf>
          </DrctDbtTxInf>
        </PmtInf>
      </CstmrDrctDbtInitn>
    </Document>
  </MsgPain008>
</conxml>

According to a bank-supplied format checking tool my file is mostly correct, except for the hash. Since PHP's implementation of SHA256 is probably not at fault I think I'm doing something wrong when canonicalizing the document.

The code in question is rather simple:

function documentHash($element)
{
    $text = $element->C14N(true, true);
    return strtoupper(hash('sha256', $text));
}

$element is the DOMElement containing the <Document> node. This function is called during the creation of the container file; thus the xmlns:xsi parameter is not applied to the node. However, manually adding it did not seem to improve matters.

The generated hash is wrong - it's 058098433DAC5D66ED34933CFFD98BF65CAD5C97CC45F9B0619B1FF96C3930E7; the expected value according to the format checker is 942AB2F57DBAF6302EDC526472098DF38C540EB75E1913DAB0DF416D168C3253. Now the question is what I'm doing wrong here - or rather how the I have to massage my XML in order to please the bank.

Here is what $text looks like: (Unfortunately, canonicalized XML does not make for pretty code blocks.)

<Document xmlns="urn:iso:std:iso:20022:tech:xsd:pain.008.003.02"><CstmrDrctDbtInitn><GrpHdr><MsgId>testmsg-1</MsgId><CreDtTm>2014-02-14T09:32:37+01:00</CreDtTm><NbOfTxs>1</NbOfTxs><CtrlSum>0.01</CtrlSum><InitgPty><Nm>TestCo</Nm></InitgPty></GrpHdr><PmtInf><PmtInfId>testmsg-1-pmt-1</PmtInfId><PmtMtd>DD</PmtMtd><BtchBookg>true</BtchBookg><NbOfTxs>1</NbOfTxs><CtrlSum>0.01</CtrlSum><PmtTpInf><SvcLvl><Cd>SEPA</Cd></SvcLvl><LclInstrm><Cd>CORE</Cd></LclInstrm><SeqTp>OOFF</SeqTp></PmtTpInf><ReqdColltnDt>2014-02-14+01:00</ReqdColltnDt><Cdtr><Nm>TestCo</Nm></Cdtr><CdtrAcct><Id><IBAN>DE54100000000000012345</IBAN></Id></CdtrAcct><CdtrAgt><FinInstnId><BIC>MARKDEF1100</BIC></FinInstnId></CdtrAgt><CdtrSchmeId><Id><PrvtId><Othr><Id>DE46ZZZ00000012345</Id><SchmeNm><Prtry>SEPA</Prtry></SchmeNm></Othr></PrvtId></Id></CdtrSchmeId><DrctDbtTxInf><PmtId><EndToEndId>testmsg-1-2</EndToEndId></PmtId><InstdAmt Ccy="EUR">0.01</InstdAmt><DrctDbtTx><MndtRltdInf><MndtId>Test-1</MndtId><DtOfSgntr>2014-02-14+01:00</DtOfSgntr></MndtRltdInf></DrctDbtTx><DbtrAgt><FinInstnId><BIC>MARKDEF1200</BIC></FinInstnId></DbtrAgt><Dbtr><Nm>Other Test</Nm></Dbtr><DbtrAcct><Id><IBAN>DE90200000000000098765</IBAN></Id></DbtrAcct><RmtInf><Ustrd>Test</Ustrd></RmtInf></DrctDbtTxInf></PmtInf></CstmrDrctDbtInitn></Document>