pocketmine\event\entity\EntityDamageEvent::setDamage PHP Method

    public function setDamage($damage, $type = self::MODIFIER_BASE)
    {
        $this->modifiers[$type] = $damage;
    }

 public function onDamage(EntityDamageEvent $event)
 {
     $player = $event->getEntity();
     $entity = $event->getEntity();
     if ($player instanceof Player && $event->getCause() === EntityDamageEvent::CAUSE_ENTITY_EXPLOSION) {
         switch (mt_rand(1, 2)) {
             case 1:
                 $event->setDamage(10);
                 break;
             case 2:
                 $event->setDamage(8);
                 break;
         }
     }
 }