pocketmine\tile\Chest::saveNBT PHP Method

    public function saveNBT()
    {
        $this->namedtag->Items = new ListTag("Items", []);
        $this->namedtag->Items->setTagType(NBT::TAG_Compound);
        for ($index = 0; $index < $this->getSize(); ++$index) {
            $this->setItem($index, $this->inventory->getItem($index));
        }
    }