php - Soap 方法返回文件作为结果

Question

结果我有以下 Web 服务定义（http://www.domain.com/WebServices/Library/Library.asmx?op=GetBookByMetaData）：

HTTP/1.1 200 OK
Content-Type: text/xml; charset=utf-8
Content-Length: length

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <GetBitStreamContentByIDResponse xmlns="http://tempuri.org/">
      <GetBitStreamContentByIDResult>base64Binary</GetBitStreamContentByIDResult>
    </GetBitStreamContentByIDResponse>
  </soap:Body>
</soap:Envelope>

该方法返回空对象：

$data = $client->GetBookByMetaData(array(
          'MetaDataID' => $identifier,
          'Username' => '****',
          'Password' => '****',
      ));

我的方法调用有什么问题？

编辑：代码：

$client = new SoapClient($account->profile_dl_ul_object_info . '?WSDL');
      $data = $client->GetBookByMetaData(array(
          'MetaDataID' => $identifier,
          'Username' => $account->profile_dl_info_username,
          'Password' => $account->profile_dl_info_password,
      ));