public function getCategoriesRelated($i = null)
    {
        $wiki_api = $this->base_api . 'action=query&prop=categories&pageids=' . $this->getId();
        $json_wapi = json_decode(file_get_contents($wiki_api), true);

        $result = $this->get_list_or_item($json_wapi['query']['pages'], 'categories', false, []);
        $categories = [];

        foreach ($result as $page_id => $items) {
            $categories[$page_id] = [];
            foreach ($items as $item) {
                $categories[$page_id][] = ltrim(strstr($item['title'], ':'), ':');
            }
        }

        return ($i === null) ? $categories : $categories[$this->article_ids[$i]];
    }