Object initialization doesn't work

I just want to initialize properly an object :

namespace App;

class Produit
{
    public $nb_serveurs;
    public $type;
    public $duree;

    public function __construct($nb_serveurs, $type, $duree){
      $this->$nb_serveurs = $nb_serveurs;
      $this->$type = $type;
      $this->$duree = $duree;
    }
}

Then in my controller :

class ProductController extends Controller
{
    public function addToCard (Request $request){
      $nb_serveurs = $request->nb_serveurs;
      $type = $request->type;
      $duree = $request->duree;
      $panier = new Panier();
      $product = new Produit($nb_serveurs, $type, $duree);
      dd($product);
      $panier->addItem($product, 1);

    }
}

The dd function give me this :

Produit {#149 ▼
  +nb_serveurs: null
  +type: null
  +duree: null
  +"2": "2"
  +"5": "5"
}

i tested out, the 3 variables are not null.. What's wrong here ? And what are the 2 last lines in the Produit object ?

via Chebli Mohamed