How can I collect result from prepare statement PHP?

I can't collect the results of my query in this function :

    require_once('connection.php');

    $idJoueur = session_id();

    if ($stmt = $conn->prepare('SELECT bestScore FROM Player WHERE ID = ?')) {
        $stmt->bind_param('i',$idJoueur);
        $stmt->execute();
        $result = $stmt->get_result();

        $data = $result->fetch_assoc();

        $bestScore = $data;

        if (!(empty($bestScore))) {
            header("Location: ../index.php");
        }

        return $bestScore;
    }
    else {
        $error = $conn->errno . ' ' . $conn->error;
        echo $error;
    }

}</code>

My connection works, I tried fetch_assoc(), store_result() but I failed every time