OpenCart 🇺🇦

$output = '';

$results = $this->model_catalog_category->getCategories($parent_id);

foreach ($results as $result) {
	if (!$current_path) {
		$new_path = $result['category_id'];
	} else {
		$new_path = $current_path . '_' . $result['category_id'];
	}

	$output .= '<url>';
	$output .= '<loc>' . $this->url->link('product/category', 'path=' . $new_path) . '</loc>';
	$output .= '<changefreq>weekly</changefreq>';
	$output .= '<priority>0.7</priority>';
	$output .= '</url>';

	$products = $this->model_catalog_product->getProducts(array('filter_category_id' => $result['category_id']));

	foreach ($products as $product) {
		$output .= '<url>';
		$output .= '<loc>' . $this->url->link('product/product', 'path=' . $new_path . '&product_id=' . $product['product_id']) . '</loc>';
		$output .= '<changefreq>weekly</changefreq>';
		$output .= '<priority>1.0</priority>';
		$output .= '</url>';
	}

	$output .= $this->getCategories($result['category_id'], $new_path);
}

return $output;