Why does my class don't understand a variable that I setted in the class

I have 2 classes, one have 2 variables, and the other have an array who will take theses 2 variables to make a list of a list (I guess, maybe it won't work) but here my problem:

ErrorException array_push() expects parameter 1 to be array, null given

<?php
namespace App\Libraries;
class Caddie 
{

    public $commandeListe = [];

    public function add(string $produit, int $quantite)
    {
        $ligne = [$produit,$quantite];
        array_push($commandeListe,$ligne); //the error is here
    }


    public function getAll()
    {
        return $this->commandeListe;
    }


};

?>

I'm not sure if the other code is useful but here it is

<?php
namespace App\Libraries;
class Ligne 
{

    public string $produit;
    public int $quantite;



    public function __construct($produit, $quantite)
    {
        $this->produit = $produit;
        $this->quantite = $quantite;
    }   


    public function getProduit()
    {
        return $this->produit;
    }
    
    
    public function getQuantite()
    {
        return $this->quantite;
    }

}

?>

I think the Caddie class don't understand $commandeListe but I don't understand why.

I use codeigniter but I don't think it as an influence on this problem

It's the first time that I ask a question so, sorry if I didn't use the website properly