code: public function set(string $key, $value): void
      {
          $key = explode('.', $key);
          $last = array_pop($key);
          $result = &$this->config;
  
          foreach ($key as $part) {
              if (!isset($result[$part]) || !is_array($result[$part])) {
                  $result[$part] = [];
              }
              $result = &$result[$part];
          }
          $result[$last] = $value;
      }
docs: Sets new value.
  
  @param string $key
  @param $value