class FooterWalker extends Walker_Nav_Menu {
    private $column_open = false;

    function start_lvl(&$output, $depth = 0, $args = []) {
        if ($depth === 0) {
            $output .= "<ul>\n";
        }
    }

    function end_lvl(&$output, $depth = 0, $args = []) {
        if ($depth === 0) {
            $output .= "</ul>\n";
        }
    }

    function start_el(&$output, $item, $depth = 0, $args = [], $id = 0) {
        // Start new column on top-level items
        if ($depth === 0) {
            if ($this->column_open) {
                $output .= "</div>\n"; // Close previous column
            }
            $output .= "<div class=\"footer-column\">\n";
            $output .= "<h4>{$item->title}</h4>\n";
            $this->column_open = true;
        } else {
            $output .= "<li><a href=\"{$item->url}\">{$item->title}</a></li>\n";
        }
    }

    function end_el(&$output, $item, $depth = 0, $args = []) {
        // Do nothing on end_el
    }

    function walk($elements, $max_depth, ...$args) {
        $output = parent::walk($elements, $max_depth, ...$args);
        if ($this->column_open) {
            $output .= "</div>\n"; // Close last column
            $this->column_open = false;
        }
        return $output;
    }
}