Php - SOAP API with auth

I encounter some problems when I try to call a method of api with authentication.

 $auth = (object) array(
                'company'=> 'companyname'
    $url = '';
    $soap = new SoapClient('',$auth);
    echo $soap->nop();

I get an error : The user parameter is empty or missing.

My question : How can I send a request xml like this :

<?xml version='1.0' encoding='UTF-8'?> 
<soapenv:Envelope xmlns:soapenv=""> 
<xs:nvheader xmlns:xs=""> 
         mot de passe

I must have this structure so that the API SOAP will work.