check.php

<?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,
]);