knockback

Okay so i am trying to knockback a player once they reach the border (knock them backwards by the way) i know i can use Living::knockBack() but when i use it(my code):

PHP:

 $player->knockBack($player, 0, $player->getX() - $player->getX(), $player->getZ() - $player->getZ(), 0.4);

nothing happens it even sends the message about the player hitting the border too(full code):

PHP:

if($this->getUtils()->getGameManager()->getStatus() === GameManager::STARTED_GRACE or $this->getUtils()->getGameManager()->getStatus() === GameManager::STARTED_PVP){
                if(round($player->distance($player->getLevel()->getSpawnLocation())) >= Utils::$world->getBorder()){
                    $player->knockBack($player, 0, $player->getX() - $player->getX(), $player->getZ() - $player->getZ(), 0.4);
                    $source = new EntityDamageEvent($player, EntityDamageEvent::CAUSE_CUSTOM, 0.1);
                    $player->attack($source);
                    $player->sendMessage(TextFormat::RED."You hit the RED ZONE");
                }
     }

 

because $player->getX() - $player->getX() is 0 and $player->getZ() - $player->getZ() is 0, so no motion is proceeded