code: private function readSpgrContainer()
      {
          // context is either context DgContainer or SpgrContainer
  
          $length = Xls::getInt4d($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
          $spgrContainer = new SpgrContainer();
  
          if ($this->object instanceof DgContainer) {
              // DgContainer
              $this->object->setSpgrContainer($spgrContainer);
          } else {
              // SpgrContainer
              $this->object->addChild($spgrContainer);
          }
  
          $reader = new self($spgrContainer);
          $escher = $reader->load($recordData);
      }
docs: Read SpgrContainer record (Shape Group Container).