NuSoap equivalent of SoapVar() - php

Question

I don't know much about NuSoap, but unfortunately I have to use it.
I'm trying to follow example code for the dotmailer web service api Here. I need to be able to add custom fields for subscribers.
In the example code custom/additional fields are defined like so;
new SoapVar($FirstName,XSD_STRING,"string","http://www.w3.org/2001/XMLSchema");
Heres the code I have right now;
<?php
function subscribeUserEmail($email)
{
$username = "********";
$password = "********";
$postURL = "http://apiconnector.com/api.asmx?WSDL";
$contact = array (
"Email" => $email,
"AudienceType" => "B2C",
"OptInType" => "Single",
"EmailType" => "Html",
"ID" => -1,
"DataFields" => array(
"Keys" => array("TEST"),
"Values" => array("Name")
)
);
$params = array(
"username" => $username,
"password" => $password,
"contact" => $contact,
"addressbookId" => "******"
);
$client = new soapclient($postURL, true);
$error = $client->getError();
$result = $client->call('AddContactToAddressBook', $params);
echo "<h2>Request</h2>";
print("<pre>".$client->request."</pre>");
echo "<h2>Response</h2>";
print("<pre>".$client->response."</pre>");
echo "<h2>Debug</h2>";
print("<pre>".$client->debug_str."</pre>");
}
?>
If the $contact array is changed to this;
$contact = array (
"Email" => $email,
"AudienceType" => "B2C",
"OptInType" => "Single",
"EmailType" => "Html",
"ID" => -1,
"DataFields" => array(
"Keys" => array("TEST"),
"Values" => array(new SoapVar("Name",XSD_STRING,"string","http://www.w3.org/2001/XMLSchema") )
)
);
And used with a regular soap client, the code works. So I'm pretty sure there aren't any other problems with my function.
I've tried using the following;
new soapval("string", XSD_STRING, "Name","http://www.w3.org/2001/XMLSchema");
As an alternative to the SoapVar() method, but I get the same errors as if I entered the value as plain text.
How can I replicate the functionality of SoapVar() in NuSoap? This appears to be the only issue.