How to to make a store tree view cart

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SQL

PHP

MySQL

PDO

I am building an MVC cart system. I have the MERCHANTS table and the PRODUCTS table. A merchant (store) can have one or more products, everything works fine but what I want is to display products by store, I mean to say that once a user adds, for example, some products that belong to the same shop, the cart should display like a treeview. I have designed two functions, one that builds the HTML view and the other function which builds the model.

What I have tried:

CREATE TABLE `cart` (
    `cartId` int(10) UNSIGNED NOT NULL,
    `product_id` int(10) UNSIGNED NOT NULL,
    `user_id` int(10) UNSIGNED DEFAULT NULL,
) ENGINE=InnoDB DEFAULT CHARSET=latin1;


CREATE TABLE `merchants` (
    `merchant_id` int(11) UNSIGNED NOT NULL,
    `merchant_name` varchar(255) DEFAULT NULL,
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE `products` (
    `product_id` int(11) UNSIGNED NOT NULL,
    `merchant_id` int(11) UNSIGNED NOT NULL,
    `product_name` varchar(255) NOT NULL,

) ENGINE=InnoDB DEFAULT CHARSET=latin1;

$userId = 1; // This is just for an example
$itemsInCart= getCart($userId); // Fetch the items in cart from the DB on session
function getCart($userId): array
{
    try {
        $db = createConnection();
        $sql = 'SELECT * FROM cart
                LEFT JOIN products p ON cart.product_id = p.product_id
                LEFT JOIN merchants m ON p.merchant_id = m.merchant_id
                WHERE user_id = :userId ';
        $stmt = $db->prepare($sql);
        $stmt->bindValue(':userId', $userId, PDO::PARAM_INT);
        $stmt->execute();
        $invInfo = $stmt->fetchAll(PDO::FETCH_ASSOC);
        $stmt->closeCursor();
        return $invInfo;
    } catch (PDOException $e) {
        exit($e->getMessage()) ;
        return [];
    }
}

$userCartDisplay = buildUserCartDisplay($itemsInCart); // Builds the cart elements' view
function buildUserCartDisplay($itemsInCart)
{
    $html = '';
    foreach($itemsInCart as $cart) {
        $html .= '<ul> Store name goes right here';
        $html .= '<li> store Products name go right here <li> ';
        $html .= '</ul>  ';
    }
    return $html;
}

echo $userCartDisplay; // Display the view