java.lang.NullPointerException on PHP soap request - php

Question

I am trying soap request with authentication. Wdsl server requiring user authentication. So i used SOAP header method.
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ser="http://service.connector.uut.cs.com.tr/">
<soapenv:Header>
<wsse:Security xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd">
<wsse:UsernameToken>
<wsse:Username>my_user_name</wsse:Username>
<wsse:Password>my_password</wsse:Password>
</wsse:UsernameToken>
</wsse:Security>
</soapenv:Header>
<soapenv:Body>
<ser:sendInvoice>
<id>123456</id>
<type>INVOICE</type>
<invoiceNo>AAA123456001</invoiceNo>
<data></data>
<hash>dfa06b85d66ef786f9f091f38e09c184</hash>
<mimeType>application/xml</mimeType>
<version>1.0</version>
</ser:sendInvoice>
</soapenv:Body>
</soapenv:Envelope>
Php soap request:
$auth = array(
'Username' => 'my_user_name',
'Password' => 'my_password',
);
$url = 'https://test.com/ws/connectorService?wsdl';
try {
$request = new SoapClient( $url, $auth );
$header = new SOAPHeader( $url, 'Security', $auth );
$request->__setSoapHeaders( $header );
$result = $request->sendInvoice();
} catch( Exception $exc ) {
$result = $exc->getMessage();
}
The $result is printing "java.lang.NullPointerException".