WSDL Soap with PHP giving error

I have the following code

    try {

$client = new SoapClient("link.wsdl");
$function = "METADOPRINCIPAL";

$arguments =  array(

'METADOPRINCIPAL' => array(
     'Msg'     => array(
       'Transacao'     => array(
         'ME_DVW' => array(
           'INSTITUCAO'     => 'K2',
           'CURSO'       => $curso
         )
        )
     )




 ));
$options = array('location' => '"link.wsdl"');
$result = $client->__soapCall($function, $arguments, $options);

print_r($result);
} 
catch (Exception $e) {
echo "Error! ";
   echo $e -> getMessage ();}

And I'm trying to get a simple get of the parameters of the METADOPRINCIPAL method but it's giving error:

  

SOAP-ERROR: Encoding: object has no 'FieldTypes' property

I have tried many ways to make this SOAP connection work and I can not, can you help me?

I need to pass only 2 parameters to get return, but it seems to need to send the entire xml

'CURSO'       => $curso / 'INSTITUICAO'       => 'K2'