带头数据的SOAP请求

I'm trying to connect to a WCF service from PHP using the SoapClient class. I can successfully connect and traverse the API using WCFStorm however cannot seem to construct the header data correctly in order to authenticate from SoapClient. A working WCF request would be as follows:

<MyMethod>
  <OutgoingHeaders attr0="MsgHeaderArray" isNull="false">
    <MsgHeaderArray0>
      <Name>ApiKey</Name>
      <Namespace>http://www.service.com/namespace</Namespace>
      <ItemType>System.String</ItemType>
      <Value>ABC123</Value>
      <Direction>Outgoing</Direction>
    </MsgHeaderArray0>
  </OutgoingHeaders>
  <MethodParameters>
    <colour>Red</colour>
    <size>Large</size>
  </MethodParameters>
</MyMethod>

The code I'm using to connect to the api and call the method is:

$params = array(
  'colour' => 'Red', 
  'size' => 'Large'
);

$service = new SoapClient('http://www.service.com/service.svc?wsdl');
$header = new SoapHeader('http://www.service.com/namespace', 'ApiKey', 'ABC123', FALSE);
$service->__setSoapHeaders(array($header));

$service->MyMethod($params);

However I'm getting an access denied error, I'm guessing because SoapHeader isn't corrently formatted?

SoapFault: Access is denied. in SoapClient->__call()

Thank you for any help you can provide.