add_action('wp', 'update_online_users_status');
function update_online_users_status(){

  if(is_user_logged_in()){

    // get the online users list
    if(($logged_in_users = get_transient('users_online')) === false) $logged_in_users = array();

    $current_user = wp_get_current_user();
    $current_user = $current_user->ID;
    $current_time = current_time('timestamp');

    if(!isset($logged_in_users[$current_user]) || ($logged_in_users[$current_user] < ($current_time - ($time_limit * 60)))){
      $logged_in_users[$current_user] = $current_time;
      set_transient('users_online', $logged_in_users, 30 * 60);
    }

  }
}


// use this function to check if a user is online
// pass the user ID
// example, for the author page:
//      if(is_user_online(get_query_var('author'))) echo 'user is online';

function is_user_online($user_id) {

  // get the online users list
  $logged_in_users = get_transient('users_online');

  // online, if (s)he is in the list and last activity was less than 15 minutes ago
  return isset($logged_in_users[$user_id]) && $logged_in_users[$user_id > (current_time('timestamp') - (15 * 60));
}