function _watchFilter() {
    if (isset($_POST['category']) && (int) $_POST['category'] > 0) {
        $_SESSION['_filter'] = (int) $_POST['category'];
        $_SESSION['_filterMSG'] = 'WHERE messages.gid = ' . (int) $_POST['category'];
    } else {
        $_SESSION['_filter'] = 0;
        $_SESSION['_filterMSG'] = '';
    }
}