PHP Soap variable null but __getLastResponse returns XML

I'm using PHP 5.5.9 Soap to access a web service and using a WSSoapClient (noted from here: how to implement ws-security 1.1 in php5) to help with the security. Not sure I can share the WSDL at this time. I'm getting a null response value when I call a function, but the __getLastResponse value has the XML data showing all the expected fields I see in SoapUI.

There are two functions defined in the WSDL that I'm calling, though I'm only seeing the issue with one. They both take the same arguments and I just copied the script and changed the function names to compare results, so I know there aren't typos, etc.

Function that works (Identity):

$params = array("IdentityRequest" => array("IdentityPersonRequest" => array("IdentityID" => $userid,"InterfaceID" => $interfaceID)));
$response = $client->__call('Identity',$params);

Function that doesn't work (IdentityInterface):

$params = array("IdentityInterfaceRequest" => array("IdentityPersonRequest" => array("IdentityID" => $userid,"InterfaceID" => $interfaceID)));
$response = $client->__call('IdentityInterface',$params);

When I use var_dump I get the expected object for the 'Identity' function output. When I use it for 'IdentityInterface' I get a NULL return. The __getLastResponse() output shows the XML as I'd expect it in both cases.

I'd seen some references that the WSDL may not be returning properly formatted XML, but don't see those types of errors when testing it in SoapUI. If I can't upload the WSDL, what would I need to look for to determine why this is happening.

Thank you in advance.