PHP NuSoap Client and ArrayOfString complexType

I’m a API newbie and I’m trying to use a nuSOAP call to a function (GetPartDetails) within an WSDL to acquire some data.

Here’s the WSDL: WSDL

I’m able to properly format the parameters within my call to get all of the needed results back aside from this one

<s:element minOccurs="0" maxOccurs="1" name="Parts" type="tns:ArrayOfString"/>

(the Parts type)

Here’s how I have it formatted in PHP:

$params		   = array (
				'StockCheckID'	=>	$stockcheckid, 
				'Parts' 		=>	array('1-179228-2'),
				'SearchType'	=>	$searchtype,
				'Region'		=>	$region,
			  );

$result 	= $client->call('GetPartDetails',$params);

Do I need to format this parameter differently, or am I missing something else?

Instead of this:

array('1-179228-2'),

do this:

json_encode(array('1-179228-2'));

Not sure why it works, but it does. I’m using the StockCheck API too.