help with array

i know this is not about pocketmine but i looked at php.net array but i didn ´t find where is the problem.

array:

PHP:

public $mapdata = ["canyon" => ["bluespawn" => new Vector3(1, 2, 3)]];
                                                          

method to get content from array:

PHP:

public function getMapData($key)
    {
        return isset($this->mapdata[$key]) ? $this->mapdata[$key] : false;
    }

i want create other method which return "bluespawn" from array
i tried this but doesn ´t work

PHP:

public $map = "canyon";

public function getArenaData($data){
        return isset($this->getMapData($this->map)[$data]) ? $this->getMapData($this->map)[$data] : false;
    }

pocketmine log: Fatal error: Can't use method return value in write context
(getArenaData is bad method)

EDIT: SOLVED

i forgot ] but i have it in my code but still doesn´t work

You probably post the solution for future readers

because i tried $this->getArenaData("bluespawn")++; xD