Accessing internalID of ListOrRecordRef in PHP

I am using the PHP Toolkit to read and update custom object data.

One of my custom objects has a field which is a "ListOrRecordRef" field (specifically, a reference to another custom object of the same type). I have pulled this data out of the record using:

$fieldRef = $custom_field->getCustomFieldValue('fieldname');

The problem is that I can't figure out what function to use to extract the data from $fieldRef. Here's what the print_r of this object looks like:

nsComplexObject Object ( [nsComplexObject_type] => ListOrRecordRef [nsComplexObject_namespace] => urn:core_2011_2.platform.webservices.netsuite.com [nsComplexObject_fields] => Array ( [internalId] => 844 [typeId] => 1 ) [nsComplexObject_namespaces] => Array ( [0] => setupcustomization [1] => platformcore ) )