pocketmine\inventory\BaseInventory::firstOccupied PHP Method

    public function firstOccupied()
    {
        for ($i = 0; $i < $this->size; $i++) {
            if (($item = $this->getItem($i))->getId() !== Item::AIR and $item->getCount() > 0) {
                return $i;
            }
        }
        return -1;
    }