Events

PHP:

<?php
namespace _0110\CallFunction;

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

class CallFunction extends PluginBase implements Listener{
    public function onEnable(){
    }
    public function onPlayerJoin(PlayerJoinEvent $event){
        $event->getPlayer()->sendMessage("Hello");
    }
}

Ok, obviously, I wrote this just to test event stuff, but nothing really happens when I join. What have I forgot?

You forgot to register the events. Add $this->getServer()->getPluginManager()->registerEvents($this, $this); into onEnable().

I already tried that, and Im getting crash for undefined method. What do I use for it?

Code?

Instead of telling us your errors, try posting your latest code.

Well that is all I have above I was trying to write a plugin, but the functions werent triggered, so I tried to write even simpler code but it didnt work. And that simpler code is that above.

What functions were you trying to use? + You need to register events.
Show the errors you were getting.

PHP:

<?php
namespace _0110\CallFunction;

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

class CallFunction extends PluginBase implements Listener{
    public function onEnable(){
        $this->getServer()->getPluginManager()->registerEvents($this, $this);
    }
    public function onPlayerJoin(PlayerJoinEvent $event){
        $event->getPlayer()->sendMessage("Hello");
    }
}

This will work.

Instead of saying "instead of", try saying " apart from". The error is important as well.