Help with a defined variable claiming to be undefined

Hey all,
I'm working on my timer for the lobby in my plugin, And I'm getting an error with my variable claiming to be undefined when the plugin runs in PM,

Code:

<?php
namespace EMC;
use pocketmine\scheduler\PluginTask;
use pocketmine\Server;
class serverTimer extends PluginTask{
    public $lobbyTime = 61;

    public function onRun($currentTick){
        Server::getInstance()->broadcastMessage(">>The next game will begin in ".$this->lobbyTime." seconds");

        if($this->$lobbyTime == 60){
            $this->timerStartMessages();

        }

        if($this->$lobbyTime < 6 && $this->$lobbyTime>0){
            $this->timerStartMessages();
        }

        if($this->$lobbyTime = 0){
            $totalPlayers = 0;

            foreach($this->getOwner()->getServer()->getOnlinePlayers() as $p){
                $totalPlayers++;
            }
            Server::getInstance()->broadcastMessage(">>Timer reset: ".$totalPlayers." online, 4 needed to play!");

            $this->$lobbyTime = 60;

        }

        $this->$lobbyTime--;
    }

    public function timerStartMessages(){
        Server::getInstance()->broadcastMessage(">>The next game will begin in ".$this->lobbyTime." seconds");

    }
}

The error starts at the line at the first line in the onRun(), claiming the variable "lobbyTime" is undefined.

Earlier the plugin would allow me to run untill the "if" statement before it gave the error.
Thanks for reading

Ps: please forgive me if I've done something real stuipid without realising. I've had a massive crash course in php and am getting alright at a lot of things, but theres just minor things i still gotta learn. Also i looked online for anyone with similar issues, but I haven't found anything so far.

remove the $

$this->lobbyTime = 60;
$this->lobbyTime--;

thank you