how can i make global $sender

oh, so this thread is actually about class properties? And not how the title suggests global vars?

 

back to the title's point.
all you have todo to make $sender global(class properties) is

PHP:

$this->sender = $sender;

if you want you can add

PHP:

public $sender;

right under the class part where you extend PluginBase.

example because am nice

PHP:

<?php

namespace name\space;

use pocketmine\plugin\PluginBase;

class YourClass extends PluginBase{

public $sender; # ;P

public function onEnable(){}

}