pocketmine\entity\Entity::setOnFire PHP Method

    public function setOnFire($seconds)
    {
        $ticks = $seconds * 20;
        if ($ticks > $this->fireTicks) {
            $this->fireTicks = $ticks;
        }
    }

 public function onEntityCollide(Entity $entity)
 {
     $entity->setOnFire(8);
     $ev = new EntityDamageEvent($entity, EntityDamageEvent::CAUSE_FIRE, 1);
     Server::getInstance()->getPluginManager()->callEvent($ev);
     if (!$ev->isCancelled()) {
         $entity->attack($ev->getFinalDamage(), $ev);
     }
 }