code: public function set($key, $value)
      {
          if ($key == '') {
              throw new InvalidArgumentException('Invalid config key.');
          }
          // 只支持三维数组，多余无意义
          $keys = explode('.', $key);
          switch (count($keys)) {
              case '1':
                  $this->config[$key] = $value;
                  break;
              case '2':
                  $this->config[$keys[0]][$keys[1]] = $value;
                  break;
              case '3':
                  $this->config[$keys[0]][$keys[1]][$keys[2]] = $value;
                  break;
              default:
                  throw new InvalidArgumentException('Invalid config key.');
          }
          return $this->config;
      }
docs: 设置配置选项
  @param string $key
  @param string $value
  @return array