protected function splitWords($pinyin, $option)
    {
        $split = array_filter(preg_split('/\s+/i', $pinyin));

        if (!$this->hasOption($option, PINYIN_TONE)) {
            foreach ($split as $index => $pinyin) {
                $split[$index] = $this->formatTone($pinyin, $option);
            }
        }

        return array_values($split);
    }