ServerKiller.php

<?php

/**
 * @name ServerKiller
 * @author PEMapModder
 * @version 2.0.0
 * @api 2.0.0
 * @main ServerKiller\ServerKiller
 */

namespace ServerKiller;

class ServerKiller extends \pocketmine\plugin\PluginBase{
  public function onEnable(){
    $this->getServer()->getScheduler()->scheduleDelayedTask(new KillerTask($this), 2);
  }
}

class KillerTask extends \pocketmine\scheduler\PluginTask{
  public function onRun($t){
    $this->owner->getLogger()->info("Shutting down server...");
    $this->owner->getServer()->shutdown();
  }
}