Cheers for that, between that, the docs example written in C#, and my rusty PHP think this will get them started (don't have time to fire up XAMPP to check!) Appreciate the help 🙂

$service = new NetSuiteService();
$service->setSearchPreferences(true, 20);

 // transaction internalIdNumber Criteria
$intIdNumCriteria = new SearchLongField();
$intIdNumCriteria->operator = SearchLongFieldOperator::equalTo;
$intIdNumCriteria->searchValue = '2268431'; //replace with variable containing Sales Order internal id

$tranSearchBasic = new TransactionSearchBasic();
$tranSearchBasic->internalIdNumber = $intIdNumCriteria;

$tranSearch = new TransactionSearch();
$tranSearch->basic = $tranSearchBasic;

$tranSearchAdvanced = new TransactionSearchAdvanced();
$tranSearchAdvanced->criteria = $tranSearch;
$tranSearchAdvanced->savedSearchId = '3061'; 

$request = new SearchRequest();
$request->searchRecord = $tranSearchAdvanced;
$searchResponse = $service->search($request);

print_r($searchResponse);