How do I override the /help command?

Some time ago, you could do

PHP:

public function onPlayerCommandPreprocess(\pocketmine\event\player\PlayerCommandPreprocessEvent $event) : void{
  $player = $event->getPlayer();
  $command = explode(" ", $event->getMessage());

  if($command === "/help"){
    $player->sendMessage("Overriding /help");
    $event->setCancelled();
   }
}

to override the /help command. The other way would be to unregister the command.

Since /help is client side, what I can do?