$mainResponse = array(
                 "result" => array(
                 ),
                 "ack" => "success"
            );

       foreach($resp->searchResult->item as $item) {
         $itemId = $item->itemId;

         $addThisItem = array("itemId" => $itemId);

     }
}
   $mainResponse['result'] = $addThisItem;
   echo json_encode($mainResponse);

{"result":{"itemId":{"0":"183851233368"}},"ack":"success"}

{"result":[{"itemId":"183851233368"},...],"ack":"success"}