How to make a button execute a php script?

Basic scenario:

Store currently authenticated user's unique id and time in a .txt file in the following format (as JSON):

json_encode(['user_id' => 1, 'created_at' => (new DateTime('now'))->format('Y-m-d H:i:s')]);

Check the stored user_id and created_at fields in the file when a user attempts to sign in.
If the file is empty, log the user in and write user's unique id and time to the file.
If the file is not empty and user_id field is same as the id of the user who attempts to log in and created_at field is not older than 12 hours ago (or your custom logic), just update created_at field in the file and log the user in.
If the file is not empty and user_id field is same as the id of the user who attempts to log in, but passed more than 12 hours (or your custom logic), ask the user if he/she want to take over another user.
If the file is not empty and user_id field is not same as the id of the user who attempts to log in ask the user if he/she want to take over another user.

Basic implementation:

Create a .txt file in your project directory.
Add these helper functions to your project (helpers.php in my case):

if (! function_exists('check_auth')) {
    function check_auth(): bool
    {
        if (! isset($_SESSION['user_id'])) {
            return false;
        }

        if (! file_exists('current_user.txt') || filesize('current_user.txt') === 0) {
            return true;
        }

        $trace = json_decode(file_get_contents('current_user.txt'));

        // You can write your own logic here.
        return (int) $trace->user_id === $_SESSION['user_id'] && (new DateTime($trace->created_at))->modify('+12 hours') > new Datetime('now');
    }
}

if (! function_exists('logout'))
{
    function logout()
    {
        if (isset($_SESSION['user_id'])) {
            $trace = json_decode(file_get_contents('current_user.txt'));

            if ((int) $trace->user_id === $_SESSION['user_id']) {
                file_put_contents('current_user.txt', '');
            }

            unset($_SESSION['user_id']);
        }
    }
}

if (! function_exists('redirect')) {
    function redirect(string $url, int $status_code = 303): void
    {
        header('Location: ' . $url, true, $status_code);
        die();
    }
}

Create a login page (login.php in my case):

<?php

declare(strict_types=1);

// Start session.
session_start();

// Include helper functions.
require_once 'helpers.php';

// Redirect user to homepage/dashboard if authenticated.
if (check_auth()) {
    redirect('index.php');
    return;
}

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    $pdo = new PDO('mysql:host=[DB_HOST];dbname=[DB_NAME];charset=utf8mb4', '[DB_USERNAME]', '[DB_PASSWORD]', [
        PDO::ATTR_ERRMODE            => PDO::ERRMODE_EXCEPTION,
        PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_OBJ,
        PDO::ATTR_EMULATE_PREPARES   => false,
    ]);
    $stmt = $pdo->prepare('SELECT * FROM users WHERE email = ?');
    $stmt->execute([$_POST['email']]);
    $user = $stmt->fetch();

    if (! ($user && password_verify($_POST['password'], $user->password))) {
        echo json_encode([
            'success' => false,
            'message' => 'These credentials don\'t match our records.',
        ]);
        return;
    }

    // Log user in if another is not authenticated.
    if (filesize('current_user.txt') === 0) {
        file_put_contents('current_user.txt', json_encode([
            'user_id'    => $user->id,
            'created_at' => (new DateTime('now'))->format('Y-m-d H:i:s'),
        ]));

        $_SESSION['user_id'] = $user->id;

        echo json_encode([
            'success' => true,
        ]);

        return;
    }

    $trace = json_decode(file_get_contents('current_user.txt'));

    // Log user in if the last authenticated user is himself/herself.
    if ((int) $trace->user_id === $user->id) {
        $trace->created_at = (new DateTime('now'))->format('Y-m-d H:i:s');

        file_put_contents('current_user.txt', json_encode($trace));

        $_SESSION['user_id'] = $user->id;

        echo json_encode([
            'success' => true,
        ]);

        return;
    }

    // Ask user if he/she wants to take over.
    echo json_encode([
        'success'  => false,
        'takeover' => true,
        'message'  => 'Another user is logged in. Do you want to take over?',
    ]);

    return;
}

?>

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Login</title>
</head>
<body>
    <form method="post" id="form">
        <div>
            <label for="email">Email:</label>
            <input type="email" id="email" name="email" placeholder="Email" required>
        </div>
        <div>
            <label for="password">Password:</label>
            <input type="password" id="password" name="password" placeholder="Password">
        </div>
        <div>
            <span id="message" style="color: red;"></span>
        </div>
        <button>Log in</button>
    </form>
    <script src="https://code.jquery.com/jquery-3.5.1.min.js"></script>
    <script>
        $(function () {
            $('#form').submit(function (e) {
                e.preventDefault();
                $('#message').text('');

                $.post('login.php', $(this).serialize(), function (response) {
                    const res = JSON.parse(response);

                    if (res.takeover) {
                        // Ask user if he/she wants to take over. If user confirms, run `confirmed()` function.
                        confirm(res.message) && confirmed();
                        return;
                    }

                    if (res.success) {
                        // Login is successful. Reload or redirect user to another page.
                        location.reload();
                    } else {
                        // Login failed. Incorrect email or password entered.
                        $('#message').text(res.message || '');
                    }
                });
            });

            function confirmed() {
                $.post('confirmed.php', function (response) {
                    const res = JSON.parse(response);
                    console.log(res.data);
                });
            }
        });
    </script>
</body>
</html>

Check if another user took over currently authenticated user in your pages (index.php in my case):

<?php

declare(strict_types=1);

// Start session.
session_start();

// Include helper functions.
require_once 'helpers.php';

?>
<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Home</title>
</head>
<body>
<?php if (check_auth()): ?>
    Welcome friend.
<?php else: ?>
    <a href="/login.php">Log in</a>
<?php endif; ?>

<script src="https://code.jquery.com/jquery-3.5.1.min.js"></script>
<script>
    $(function () {
        // Check if another user took over currently authenticated user in every 3 seconds.
        setInterval(function () {
            $.post('check.php', function (response) {
                let res = JSON.parse(response);
                if (! res.auth && res.redirect) {
                    location.replace(res.redirect);
                }
            });
        }, 3000);
    });
</script>
</body>
</html>

Implement your check logic (check.php in my case):

<?php

declare(strict_types=1);

// Start session.
session_start();

// Include helper functions.
require_once 'helpers.php';

if (! check_auth()) {
    logout();

    echo json_encode([
        'auth'     => false,
        'redirect' => 'login.php',
    ]);

    return;
}

echo json_encode([
    'auth' => true,
]);

Finally, create your action/function for the case when user confirms to take over (confirmed.php in my case):

<?php

declare(strict_types=1);

// Start session.
session_start();

// Include helper functions.
require_once 'helpers.php';

/**
 * Run your action here if user confirms to take over another user.
 */
echo json_encode([
    'data' => 'User confirmed to take over another user.',
]);

Tested and works fine. But you should customize it for your needs.

Here the basic concept.

You need a central storage where the current active user active_user_id and last_updated is stored (database/file/memcache/redis whatever you want)
You need to poll the current active user for each session - if the current user is not the active user he will logged out and notified, so he knows someone else took over.

Flow

User loggs in
- $_SESSION['user_id'] = user_id
- IF: $_SESSION['user_id'] !== active_user_id(central storage - see above) AND last_updated < 10s
  - TRUE: log in, update active_user_id to $_SESSION['user_id']; update last_updated to now; redirect to main-page
  - FALSE: show popup "Take over?" -> ok: same as TRUE from above, abort: close popup
While logged-in call check-active.php every 5 seconds

check-active.php (pseudocode):

IF: user_id from $_SESSION['user_id'] === active_user_id from storage
- TRUE: update last_updated in storage; return 'ok';
- FALSE: call session_destroy(); return 'failed';

frontend pseudocode (logged in):

call check-active.php every 5 seconds
handle result
IF: result === 'ok'
- TRUE: do nothing
- FALSE: redirect to main page like 'main.php?message=' . urlencode('You got replaced')

Why check for last_updated > 10s?

Because someone logged in could just close the browser, so we dont know if the session is still active or not. So we check last_updated (which will be updated every 5 seconds as long as the browser is still open). If last_updated is > 10s we consider it as "noone is currently active" so we dont have to ask to take over.

Working example

Open this - hit "run" (if not running).

Then open this twice. One in a incognito tab, to get a secondary session to kick yourself out.

Disclaimer: This is absolutely experimental.

How to make a button execute a php script?

Basic scenario:

Basic implementation:

Working example

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