Users Online Count in the Admin Bar

// Minutes a user counts as online after their last page view.
$wpide_online_window = 5;

add_action( 'init', function () use ( $wpide_online_window ) {
    $user_id = get_current_user_id();

    if ( ! $user_id || wp_doing_cron() || wp_doing_ajax() ) {
        return;
    }

    $online = get_transient( 'wpide_users_online' );
    $online = is_array( $online ) ? $online : array();
    $now    = time();

    if ( isset( $online[ $user_id ] ) && $now - $online[ $user_id ] < MINUTE_IN_SECONDS ) {
        return; // Updated less than a minute ago.
    }

    $online[ $user_id ] = $now;
    $online = array_filter( $online, function ( $time ) use ( $now, $wpide_online_window ) {
        return $now - $time < $wpide_online_window * MINUTE_IN_SECONDS;
    } );

    set_transient( 'wpide_users_online', $online, HOUR_IN_SECONDS );
} );

add_action( 'admin_bar_menu', function ( $wp_admin_bar ) use ( $wpide_online_window ) {
    if ( ! current_user_can( 'list_users' ) ) {
        return;
    }

    $online = get_transient( 'wpide_users_online' );
    $online = is_array( $online ) ? $online : array();
    $now    = time();
    $online = array_filter( $online, function ( $time ) use ( $now, $wpide_online_window ) {
        return $now - $time < $wpide_online_window * MINUTE_IN_SECONDS;
    } );

    $wp_admin_bar->add_node( array(
        'id'     => 'wpide-online',
        'parent' => 'top-secondary',
        'title'  => sprintf( '%d online', count( $online ) ),
    ) );

    foreach ( array_slice( array_keys( $online ), 0, 20 ) as $user_id ) {
        $user = get_userdata( $user_id );
        if ( $user ) {
            $wp_admin_bar->add_node( array(
                'id'     => 'wpide-online-' . $user_id,
                'parent' => 'wpide-online',
                'title'  => esc_html( $user->display_name ),
                'href'   => get_edit_user_link( $user_id ),
            ) );
        }
    }
}, 5 );