ProcessWire: Restrict logins for users so that one user cannot be loggedin in more than once simultaneously

restrict-same-user-session.php

<?php namespace ProcessWire;
// code goes in ready.php
// Hook into login/logout sessions
wire()->addHookAfter('Session::loginSuccess', null, 'checkLoggedIn');
wire()->addHookBefore('Session::logout', null, 'removeLoggedIn');// Hook before to get $user->id

/**
 * Check if a user is already logged in
 *
 * If user logged in, take an action (notify,logout,etc).
 * Else, cache user as logged in to check for duplicate logins.
 * 
 * @param HookEvent $event The object (Session::loginSuccess) we are hooking into.
 * @return void
 * 
 */
function checkLoggedIn(HookEvent $event) {
    
    $user = $event->arguments('user');
    $session = wire('session');

    $userDuplicateLogin = checkUserDuplicateLogin($user);// returns boolean
    // if user logged in, do something. Here, we log them out and redirect to home page
    // you could make an exception for Superusers, or exception by role, permission, etc
    if($userDuplicateLogin) {
        $session->logout();
        $session->redirect('/');
     }
     // set cache
     else setLoggedInUserCache($user);
    /* @note: testing only
    $log = wire('log');
    $log->save("user-logs","Successful login for '$user->name'"); */

}

/**
 * Check if a user is logged in more than once.
 *
 * @param User $user The user to whose logins to check.
 * @return Boolean $duplicateLogIn True if user already logged in, else false.
 * 
 */
function checkUserDuplicateLogin(User $user) {    
    $cache = wire('cache');
    $duplicateLogIn = false;
    $userID = $user->id;    
    $cachedUsersIDs = $cache->get('loggedInUserIDs');// array OR null
    if(is_array($cachedUsersIDs) && isset($cachedUsersIDs[$userID])) $duplicateLogIn = true;
    return $duplicateLogIn;
}

/**
 * Create or update cache for logged in user.
 *
 * @param User $user The user whose cache to set.
 * @return void
 * 
 */
function setLoggedInUserCache(User $user) {
    
    $cache = wire('cache');
    $userID = $user->id; 

    $cachedUsersIDs = $cache->get('loggedInUserIDs');
    // cache does not exist, create empty array ready to cache
    if(!count($cachedUsersIDs) || is_null($cachedUsersIDs)) $cachedUsersIDs = array();

    // save/update cache
    // for value, can use whatever, even $user->name; doesn't matter, key is the important thing here 
    // outer array: we use $user->id to group same user;
    // in inner array, we use session_id() to ensure uniqueness when removing cache
    $cachedUsersIDs[$userID][session_id()] = $userID;
    $cachedUsersIDsStr = json_encode($cachedUsersIDs);// JSON to save as cache
    $cache->save('loggedInUserIDs',$cachedUsersIDsStr);// @note: set expiration of cache if you wish to
    
}

/**
 * Remove a logged out user's cache.
 * 
 * This to allow subsequent logins.
 *
 * @param HookEvent $event The object (Session::logout) we are hooking into.
 * @return void
 * 
 */
function removeLoggedIn(HookEvent $event) {
    $user = wire('user');
    removeLoggedInUserCache($user);
    /* @note: for testing only
     $log = wire('log');
    $log->save("user-logs","Successful logout for '$user->name'"); */
}

/**
 * Remove the cache for a user who has logged out.
 *
 * @param User $user The user whose logged-in cache we are removing.
 * @return void
 * 
 */
function removeLoggedInUserCache(User $user) {

    $cache = wire('cache');
    $userID = $user->id;   
    
    $cachedUsersIDs = $cache->get('loggedInUserIDs');

    // cache does not exist/empty, nothing to do
    if(!count($cachedUsersIDs) || is_null($cachedUsersIDs)) return;      

    // save/update cache
    // @note: we check for current logged in user but we remove the whole user group (outer array)
    // this is because the user logged in 'validly' is logging out.
    if(isset($cachedUsersIDs[$userID][session_id()])) unset($cachedUsersIDs[$userID]);   
    $cachedUsersIDsStr = json_encode($cachedUsersIDs);
    // save updated cached
    $cache->save('loggedInUserIDs',$cachedUsersIDsStr);// @note: set expiration of cache if you wish to

}