PHP SoapClient WSDL DeserializationFailed Invalid enum value - php

Question

I am trying to use SoapClient in PHP to retrieve data from a WSDL. If I use hardcoded values, then it works with zero errors:
$client = new SoapClient(someWSDLUrl);
$params2 = array();
$params2['AuthenticationKey'] = $obj->{'LoginResult'};
$params2['VehicleClass'] = 'UsedCar';
$params2['ApplicationCategory'] = 'Consumer';
$params2['VersionDate'] = '2016';
$result2 = $client->GetMakes($params2);
I am getting the following error when I use variables or dynamic content:
Fatal error: Uncaught SoapFault exception: [a:DeserializationFailed
] The formatter threw an exception while trying to deserialize the message: There was an error while trying to deserialize parameter
http://someURL/VehicleInformationService:VehicleClass
. The InnerException message was 'Invalid enum value 'UsedCars'
I have tried
$client = new SoapClient(someWSDLUrl);
$params2 = array();
$params2['AuthenticationKey'] = $obj->{'LoginResult'};
$params2['VehicleClass'] = $VehicleClass;
$params2['ApplicationCategory'] = $ApplicationCategory;
$params2['VersionDate'] = $VersionDate;
$result2 = $client->GetMakes($params2);
I have also tried
$client = new SoapClient(someWSDLUrl);
$params2 = array();
$params2['AuthenticationKey'] = (string) $obj->{'LoginResult'};
$params2['VehicleClass'] = (string) $VehicleClass;
$params2['ApplicationCategory'] = (string) $ApplicationCategory;
$params2['VersionDate'] = (string) $VersionDate;
$result2 = $client->GetMakes($params2);
I have also tried:
$SubmitData = new stdClass();
$SubmitData->AuthenticationKey = new SoapVar($obj->{'LoginResult'}, XSD_STRING);
$SubmitData->VehicleClass = new SoapVar($VehicleClass, XSD_STRING);
$SubmitData->ApplicationCategory = new SoapVar($ApplicationCategory, XSD_STRING);
$SubmitData->VersionDate = new SoapVar($VersionDate, XSD_STRING);
$result2 = $client->GetMakes($SubmitData);
Same error each time. I do not know how to get around "Invalid enum value" error
Thanks in advance