public function getPopularTags()
    {
        $question = $this->findAll();

        $tagsMultiArray = array_map(function ($question) {
            return explode(',', $question->tags);
        }, $question);

        // Merge multi array, count values, sort high to low
        $tagArr = array_count_values(call_user_func_array("array_merge", $tagsMultiArray));
        arsort($tagArr);
        return $tagArr;
    }