pocketmine\event\entity\EntityRegainHealthEvent::setAmount PHP Method

    public function setAmount($amount)
    {
        $this->amount = $amount;
    }

 public function e_onRegainHealth(EntityRegainHealthEvent $event)
 {
     $player = $event->getEntity();
     if ($player instanceof Player and $event->getRegainReason() === EntityRegainHealthEvent::CAUSE_EATING) {
         if ($player->getInventory()->getItemInHand()->getId() === Item::CARROT) {
             $event->setAmount(3);
         }
     }
 }