change order status php

Hi,

I'm trying to change my po order status using php toolkit of suitetalk, but my code isn't working.

below is my code

 
  $POId = 12321;
  $PORecord = new nsComplexObject('PurchaseOrder');
 
  $POFields = array(
  'internalId' => $POId,
  'orderStatus' => '_pendingReceipt');
 
  $PORecord -> setFields($POFields);
 
  $updateResponse = $myNSclient->update($PORecord);
  if ($updateResponse->isSuccess) {
   echo 'Update Successful';
  }
  else{
   echo 'Update Unsuccessful';
  }


any help would be much appreciated.