/**
  *	get a currency id and exchange rate it by unit library
  **/
 private function update_a_exchange_rate($unit_id, $currency_code)
 {
     $res = wpl_units::update_a_exchange_rate($unit_id, $currency_code);
     $success = $res ? true : false;
     $response = array('success' => $success, 'res' => $res);
     echo json_encode($response);
     exit;
 }