Issue with a SOAP Call in PHP - Can't figure out the correct structure for the parameters

Hi I am trying to send a PHP SOAP call to create a customer in our customerdatabase (in Plentymarkets).

wsdl: http://ift.tt/1lOtaec

documentation of the fuction: http://ift.tt/1stIp3z

However I really can't figure out how to add the required parameters to the call. I used a simpler function ($soapclient->SearchOrders(array('OrderID' => '288064')); and it worked perfectly.

I tried the following but it failed:

$data = array(
    'City' => 'Hamburg',
    'CountryID' => '2',
    'CustomerNumberCity' => '2145895',
    'Email' => 'test@test.test',
    'FirstName' => 'testing',
    'HouseNo' => '15',
    'Language' => 'german',
    'Street' => 'teststr.',
    'Surname' => 'tester',
    'ZIP' => '12345',
);

$Customers = new stdClass();
$Customers->PlentySoapObject_AddCustomersCustomer = $data;

var_dump($soapclient->AddCustomers($Customers));

It returned:

object(stdClass)#6 (2) {
  ["Success"]=>
  bool(false)
  ["ResponseMessages"]=>
  object(stdClass)#7 (0) {
  }
}

GetLastRequest returned the following body:

<SOAP-ENV:Body>
    <ns1:AddCustomers>
        <oAddCustomersRequest xsi:type="ns1:PlentySoapRequest_AddCustomers">
            <Customers xsi:nil="true"/>
        </oAddCustomersRequest>
    </ns1:AddCustomers>
</SOAP-ENV:Body>