<form action="index.php?page=cart" method="post">
<input type="hidden" name="name" value="<?=$product['name']?>">

           <input type="number" name="quantity" value="1" min="1" max="<?=$product['quantity']?>" placeholder="Quantity" required>
           <input type="hidden" name="product_id" value="<?=$product['id']?>">
<select name="color">
<option value="Blue">Blue</option>
<option value="Red">Red</option>
<option value="Yellow">Yellow</option>
</select>

           <input type="submit" value="Add To Cart" name="submit">
       </form>


Pagina cu cosul de cumparaturi


echo $color = $_POST['color'];

$tva=10.00;

if (isset($_POST['product_id'], $_POST['quantity'], $color) && is_numeric($_POST['product_id']) && is_numeric($_POST['quantity'])) {

   $product_id = (int)$_POST['product_id'];
   $quantity = (int)$_POST['quantity'];


   $stmt = $pdo->prepare('SELECT * FROM products WHERE id = ?';
   $stmt->execute([$_POST['product_id']]);

   $product = $stmt->fetch(PDO::FETCH_ASSOC);

   if ($product && $quantity > 0) {

       if (isset($_SESSION['cart']) && is_array($_SESSION['cart'])) {
           if (array_key_exists($product_id, $_SESSION['cart'])) {

               $_SESSION['cart'][$product_id] += $quantity;

           } else {

               $_SESSION['cart'][$product_id] = $quantity;


           }
       } else {

           $_SESSION['cart'] = array($product_id => $quantity); // aici am incercat sa mai adaug culoare gen => array('quantity' => $quantity, 'color' => $_POST['color']));

       }
   }
}