pocketmine\item\Item::setNamedTag PHP Method

    public function setNamedTag(CompoundTag $tag)
    {
        if ($tag->getCount() === 0) {
            return $this->clearNamedTag();
        }
        $this->cachedNBT = $tag;
        $this->tags = self::writeCompoundTag($tag);
        return $this;
    }