protected function buildSoapHeader($function)
  {
    $timeStamp = $this->getTimestamp();
    $signature = $this->buildSignature($function . $timeStamp);

    return array(
      new SoapHeader(
        'http://security.amazonaws.com/doc/2007-01-01/',
        'AWSAccessKeyId',
        $this->requestConfig['accessKey']
      ),
      new SoapHeader(
        'http://security.amazonaws.com/doc/2007-01-01/',
        'Timestamp',
        $timeStamp
      ),
      new SoapHeader(
        'http://security.amazonaws.com/doc/2007-01-01/',
        'Signature',
        $signature
      )
    );
  }