pocketmine\inventory\Inventory::firstEmpty PHP Method

    public function firstEmpty();

 /**
  * @param Inventory $inventory
  */
 public function addToInventory(Inventory $inventory)
 {
     foreach ($this->getItems() as $i) {
         $inventory->setItem($inventory->firstEmpty(), clone $i);
     }
     // call_user_func_array($inventory->addItem(), $this->getItems());
 }