static function update_subscriber_meta($subscriber_id, $meta_key, $meta_value, $list_id = false)
 {
     global $wpdb;
     $lists = SendPress_Option::get('pro_notification_lists');
     $pnid = $lists['post_notifications']['id'];
     $app_only = 0;
     if ($pnid !== $list_id && $meta_key === 'post_notifications') {
         return;
     }
     if ($meta_key === 'post_notifications') {
         $app_only = 1;
     }
     $meta_table = SendPress_Data::subscriber_meta_table();
     $has_data = SendPress_Data::get_subscriber_meta($subscriber_id, $meta_key, $list_id, true);
     if (empty($has_data)) {
         return SendPress_Data::add_subscriber_meta($subscriber_id, $meta_key, $meta_value, $list_id, $app_only);
     } else {
         return $wpdb->update($meta_table, array('meta_value' => $meta_value), array('subscriberID' => $subscriber_id, 'meta_key' => $meta_key, 'meta_value' => $has_data[0]->meta_value, 'app_only' => $app_only));
     }
 }
 static function update_subscriber_meta($subscriber_id, $meta_key, $meta_value, $list_id = false)
 {
     global $wpdb;
     $meta_table = SendPress_Data::subscriber_meta_table();
     $has_data = SendPress_Data::get_subscriber_meta($subscriber_id, $meta_key, $list_id, true);
     if (empty($has_data)) {
         return SendPress_Data::add_subscriber_meta($subscriber_id, $meta_key, $meta_value, $list_id);
     } else {
         return $wpdb->update($meta_table, array('meta_value' => $meta_value), array('subscriberID' => $subscriber_id, 'meta_key' => $meta_key, 'meta_value' => $has_data[0]->meta_value));
     }
 }