public static function send_notification($alert, $url, $image_url, $app_key, $app_secret, $device_tokens)
 {
     $alert = Roost::filter_string($alert);
     $remote_content = array('alert' => $alert);
     if (null === $remote_content['alert']) {
         $remote_content['alert'] = '';
     }
     if ($url) {
         $remote_content['url'] = $url;
     }
     if ($image_url) {
         $remote_content['imageURL'] = $image_url;
     }
     if ($device_tokens) {
         $remote_content['device_tokens'] = $device_tokens;
     }
     $remote_data = array('method' => 'POST', 'remoteAction' => 'push', 'appkey' => $app_key, 'appsecret' => $app_secret, 'remoteContent' => json_encode($remote_content));
     $response = self::decode_data($remote_data);
     return $response;
 }