For screenviews after setting metrics and dimensions

$metrics    =   'ga:screenviews';
$optParams  =   array('dimensions' => 'ga:screenName');

$service = new GOOGLE_AnalyticsService($client);
try {
    $result = $service->data_ga->get(
        $GA_VIEW_ID,$start_date,$end_date,$metrics,$optParams
    );
} catch(Exception $e) {
    var_dump($e);
}

我得到了正确的结果.
我想获得实时数据但无法获得它.我试过如下.

$metrics =’rt：medium’;
$optParams = array(‘dimensions’=>’rt：activeUsers’);

try {
  $results = $service->data_realtime->get(
      $GA_VIEW_ID,'rt:activeUsers',$optParams);
} catch (apiServiceException $e) {
  // Handle API service exceptions.
  $error = $e->getMessage();
}

// Error  : Undefined PRoPErty: Google_AnalyticsService::$data_realtime

请给我解决方案以获取实时数据.