PHP – Fatal error: Unsupported operand types

I keep getting the following error and I was wondering on how to fix?

Fatal error: Unsupported operand types in C:\wamp\www\tuto\core\Controller.php on line 23

Here is line 23.

$this->vars +=  $key;

Here is the full code below.

public function set($key,$value=null){

    if(is_array($key)){

        $this->vars +=  $key;

    }else{
        $this->vars[$key]= $value;
    }
}