if ($_SERVER['REQUEST_METHOD'] === 'POST' &&
        isset($_POST['email']) &&
        isset($_POST['password'])) {

    $email = $_POST['email'];
    $password = $_POST['password'];

    $loggedUser = User::loadUserByEmail($conn, $email);

    if ($loggedUser != null) {

        $hash = $loggedUser->getHashedPassword();

        if (password_verify($password, $hash)) {

            $loggedUserId = $loggedUser->getId();

            $_SESSION['loggedUserId'] = $loggedUserId;

            header('Location: main.php');
        } else {

            $info = "Niepoprawny login lub hasło. Spróbuj ponownie";
        }
    } else {

        $info = "Niepoprawny login lub hasło. Spróbuj ponownie";
    }
}


-------------

if (!isset($_SESSION['loggedUserId'])) {

    header('Location: login.php');
} else {

    $loggedUserId = $_SESSION['loggedUserId'];

    $loggedUser = User::loadUserById($conn, $loggedUserId);
}