What could cause session variables to disappear without using unset() or session_destroy() in PHP?

Session variables could disappear without using unset() or session_destroy() in PHP if the session expires due to inactivity or if the session data is cleared by the server for some reason. To prevent this, you can set the session cookie parameters to have a longer expiration time or use session_regenerate_id() to refresh the session ID periodically.

// Set session cookie parameters with longer expiration time
session_set_cookie_params(3600); // 1 hour
session_start();

// Or use session_regenerate_id() to refresh the session ID periodically
if (isset($_SESSION['last_activity']) && time() - $_SESSION['last_activity'] > 3600) {
    session_regenerate_id(true);
    $_SESSION['last_activity'] = time();
}