PHP Toolkit retrieve Item Fulfillment record attached to a Sales Order

As the subject says, I would like to be able to retrieve the Item Fulfillment records attached to a Sales Order record, if any, using the PHP Toolkit.

The code below will retrieve the Sales Order and Cash Sale records based on the value of a custom field, but I cannot get it to include an Item Fulfillment record. Any help will be greatly appreciated.


$ts = new TransactionSearch();
$tsb = new TransactionSearchBasic();
$csf = new SearchStringCustomField();
$csf->internalId = '69';
$csf->searchValue = '100000137';
$csf->operator = 'is';
$scfl = new SearchCustomFieldList();
$scfl->customField = array($csf);
$tsb->customFieldList = $scfl;
$ts->basic = $tsb;
$request = new SearchRequest();
$request->searchRecord = $ts;
$searchResponse = $service->search($request);

Enjoy!