WSDL解释超操作参数

我正在尝试调用这个操作getCountries,但我无法理解读取WSDL文件需要哪些参数以及以何种结构化方式:

http://webservice.nizacars.es/Rentway_WS/getCountries.asmx?WSDL

我已经尝试过:

$this->soap_client->getCountries(
  array(
    'countriesRequest' => array(
      'companyCode' => $this->login,
      'allCountries' => true
     )
  )
)

$this->soap_client->getCountries(
  array(
      'companyCode' => $this->login,
      'allCountries' => true
  )
)


$this->soap_client->getCountries(
      'companyCode' => $this->login,
      'allCountries' => true
)    

但我似乎不符合规格,因为我得到了一个“ [服务器无法处理请求。-->对象引用未设置为对象的实例。] "

SoapClient的最终请求::__getLastRequest为:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ns1="http://www.jimpisoft.pt/Rentway_Reservations_WS/getCountries">
<SOAP-ENV:Body>
<ns1:getCountries/>
</SOAP-ENV:Body>
</SOAP-ENV:Envelope>

编辑,解决方案 :

$data = array(
                'getCountries' => array(
                    'objRequest' => array(
                        'companyCode' => $this->login,
                        'allCountries' => true
                    )
                )
            );
$result = @$this->_client->__call('getCountries',$data);