In this post we will see how to set customfield list in NetSuite using PHP Suitelet

$so = new SalesOrder();
$so->entity = new RecordRef();
$so->entity->internalId = 1694;
$so->itemList = new SalesOrderItemList();

$aSelectField = new SelectCustomFieldRef();
$aSelectField->value = new ListOrRecordRef();
$aSelectField->value->internalId = 1; // Internal Id of the list field value
$aSelectField->scriptId = 'custbody_order_type'; // Internal id of the select field 

$customFieldList = new CustomFieldList();
$customFieldList->customField[] = $aSelectField;
$so->customFieldList =$customFieldList;


$soi = new SalesOrderItem();
$soi->item = new RecordRef();
$soi->item->internalId = 3673;
$soi->quantity = 3;
$soi->price = new RecordRef();
$soi->price->internalId = $id;
$soi->amount = 55.3;
$so->itemList->item = array($soi);

$request = new AddRequest();
$request->record = $so;

$addResponse = $service->add($request);

Hope this resolves your issue.