code: public function getNested($keyString, $default = null, $separator = '.')
      {
          $keys = explode($separator, $keyString);
          $data = $this->array;
          foreach ($keys as $key) {
              if(!is_array($data) or !array_key_exists($key, $data)) {
                  return $default;
              }
              $data = $data[$key];
          }
          return $data;
      }
docs: Access to value if source array is nested.
  
  @param $keyString
  @param null $default default value if no nested keys has been matched
  @param string $separator
  @return array|mixed|null