get keys, no value, from mutl dem php array

    private function array_keys_multi(array $array)
    {
        $results = [];
        foreach ($array as $key => $value) {
            if (is_array($value)) {
                $results = array_merge($results,  [$key => array_merge(array_keys($value),$this->array_keys_multi($value))]);
            }
        }
        return $results;
    }