code: private function readDgContainer()
      {
          $length = Xls::getInt4d($this->data, $this->pos + 4);
          $recordData = substr($this->data, $this->pos + 8, $length);
  
          // move stream pointer to next record
          $this->pos += 8 + $length;
  
          // record is a container, read contents
          $dgContainer = new DgContainer();
          $this->object->setDgContainer($dgContainer);
          $reader = new self($dgContainer);
          $escher = $reader->load($recordData);
      }
docs: Read DgContainer record (Drawing Container).