XML请求通过soap在服务器上发送为null

Question

我的代码

$xml="<SOAP-ENV:Envelope xmlns:SOAP-ENV='http://schemas.xmlsoap.org/envelope/'>
<SOAP-ENV:Header/>
<SOAP-ENV:Body>
<FlightAvailability xmlns=''>
<strUserId>asd</strUserId>
<strPassword>asdf</strPassword>
<strAgencyId>dfs</strAgencyId>
<strSectorFrom>sdf</strSectorFrom>
<strSectorTo>PKRdsf</strSectorTo>
</FlightAvailability>
</SOAP-ENV:Body>
</SOAP-ENV:Envelope>";
$client = new SoapClient('wsdl');

$result = $client->FlightAvailability($xml);

但是服务器正在接收空值。

strUserID = null strPassword = null strAgencyId = null strSectorFrom = n ull strSectorTo = null strFlightDate = null strReturnDate = null strTripType = null strNationality = null intAdult = null intChild = null