code: private function readSpContainer()
      {
          $length = Xls::getInt4d($this->data, $this->pos + 4);
          $recordData = substr($this->data, $this->pos + 8, $length);
  
          // add spContainer to spgrContainer
          $spContainer = new SpContainer();
          $this->object->addChild($spContainer);
  
          // move stream pointer to next record
          $this->pos += 8 + $length;
  
          // record is a container, read contents
          $reader = new self($spContainer);
          $escher = $reader->load($recordData);
      }
docs: Read SpContainer record (Shape Container).