WordPress: Find Users by Last Login Activity

example.php

<?php 
/*
Plugin Name: User Activity
Plugin URI: 
Description: Provides means to query users by last login activity.
Version: 1.0
Author: Eric Teubert, Ralf Albert
Author URI: ericteubert@googlemail.com, neun12@googlemail.com
License: MIT
*/

namespace UserActivity;

add_action( 'wp_login' , '\UserActivity\add_login_time' );

function add_login_time( $user_login ) {
	global $wpdb;
	$sql = $wpdb->prepare( "SELECT ID FROM $wpdb->users WHERE user_login = %s", $user_login );
	$user_id = $wpdb->get_var( $sql );
	update_user_meta( $user_id, 'last_activity', current_time( 'mysql' ) );
}

add_action( 'auth_cookie_valid', '\UserActivity\stay_logged_in_users' );

function stay_logged_in_users( $user ){
	add_login_time( $user['username'] );
}

register_activation_hook( __FILE__, '\UserActivity\add_last_activity_for_all_users' );

function add_last_activity_for_all_users() {
	global $wpdb;
	
	$sql = $wpdb->prepare( "
		SELECT
			u.ID
		FROM
			$wpdb->users AS u
			LEFT JOIN $wpdb->usermeta m ON u.ID = m.user_id AND m.meta_key = 'last_activity'
		WHERE
			m.meta_value IS NULL" );
	$userids = $wpdb->get_col( $sql );
	
	if ( $userids ) {
		foreach ( $userids as $userid ) {
			update_user_meta( $userid, 'last_activity', current_time( 'mysql' ) );
		}
	}
}

/**
 * Find user ids from user who were inactive for a certain amount of days.
 * 
 * @param int $days
 * @return array list of user ids.
 */
function find_inactive_user_ids( $days = 30 ) {
	global $wpdb;

	$query = $wpdb->prepare( "
		SELECT
			u.ID
		FROM
			$wpdb->users AS u
			LEFT JOIN $wpdb->usermeta m ON u.ID = m.user_id AND m.meta_key = 'last_activity'
		WHERE
			m.meta_value < %s", date( 'Y-m-d', strtotime( '-' . $days . ' days' ) ) );
			
	return $wpdb->get_col( $query );
}