Problème avec array_keys

Bonjour, je suis entrain d'essayé de créé un panier depuis ma base de donnée, les données sont récupéré mais n'arrive pas a s'affiché et l'erreur qu'il m'affiche c'est : Warning: array_keys() expects parameter 1 to be array, string given in D:\Site\panierPHP\panier.php on line 27. pour moi il récupère l'id 0,1,2,3 etc... non!?
voici le code

<?php require 'header.php'; ?>
<div class="checkout">
    <div class="title">
        <div class="wrap">
        <h2 class="first">Shopping Cart</h2>
        </div>
    </div>
    <form method="post" action="panier.php">
    <div class="table">
        <div class="wrap">

            <div class="rowtitle">
                <span class="name">Product name</span>
                <span class="price">Price</span>
                <span class="quantity">Quantity</span>
                <span class="subtotal">Prix avec TVA</span>
                <span class="action">Actions</span>
            </div>

            <?php

$search_array = array('premier' => 1, 'second' => 4);
if (array_key_exists('premier', $search_array)) {
    echo "L'élément 'premier' existe dans le tableau";
}

            $ids = array_keys($_SESSION['panier']);
            if(empty($ids)){
                $products = array();
            }else{
                $products = $DB->query('SELECT * FROM accesoires WHERE id IN ('.implode(',',$ids).')');
            }
            foreach($products as $product):
            ?>
            <div class="row">
                <a href="#" class="img"> <img src="img/<?= $product->id; ?>.jpg" height="53"></a>
                <span class="name"><?= $product->Articles; ?></span>
                <span class="price"><?= number_format($product->Prix_site,2,',',' '); ?> €</span>
                <span class="quantity"><input type="text" name="panier[quantity][<?= $product->id; ?>]" value="<?= $_SESSION['panier'][$product->id]; ?>"></span>
                <span class="subtotal"><?= number_format($product->Prix_site * 1.196,2,',',' '); ?> €</span>
                <span class="action">
                    <a href="panier.php?delPanier=<?= $product->id; ?>" class="del"><img src="img/del.png"></a>
                </span>
            </div>
            <?php endforeach; ?>
            <div class="rowtotal">
                Grand Total : <span class="total"><?= number_format($panier->total() * 1.196,2,',',' '); ?> € </span>
            </div>
            <input type="submit" value="Recalculer">
        </div>
    </div>
    </form>
</div>
<?php require 'footer.php'; ?>

et voici les classe panier

<?php
class panier{

    private $DB;

    public function __construct($DB){
        if(!isset($_SESSION)){
            session_start();
        }
        if(!isset($_SESSION['panier'])){
            $_SESSION['panier'] = array();
        }
        $this->DB = $DB;

        if(isset($_GET['delPanier'])){
            $this->del($_GET['delPanier']);
        }
        if(isset($_POST['panier']['quantity'])){
            $this->recalc();
        }
    }

    public function recalc(){
        foreach($_SESSION['panier'] as $product_id => $quantity){
            if(isset($_POST['panier']['quantity'][$product_id])){
                $_SESSION['panier'][$product_id] = $_POST['panier']['quantity'][$product_id];
            }
        }
    }

    public function count(){
        return array_sum($_SESSION['panier']);
    }

    public function total(){
        $total = 0;
        $ids = array_keys($_SESSION['panier']);
        if(empty($ids)){
            $products = array();
        }else{
            $products = $this->DB->query('SELECT id, Prix_site FROM accesoires WHERE id IN ('.implode(',',$ids).')');
        }
        foreach( $products as $product ) {
            $total += $product->Prix_site * $_SESSION['panier'][$product->id];
        }
        return $total;
    }

    public function add($product_id){
        if(isset($_SESSION['panier'][$product_id])){
            $_SESSION['panier'][$product_id]++;
        }else{
            $_SESSION['panier'][$product_id] = 1;
        }
    }

    public function del($product_id){
        unset($_SESSION['panier'][$product_id]);
    }

}

si quelqu'un pourait m'expliquer mes erreurs
merci,