update product quantity in cart php session

Question

I develop a new ecommerce site using PHP. I manage cart related data to database and store that data in session. The issue is when I add a product if the product is not in cart it adds that but if a product already exists it doesn't update quantity. This is my code for reference. I use key as product id

 
    $getUserTemp['products']=array();
    $mode=mysqli_real_escape_string($conn,$_POST['mode']);
    $productId=mysqli_real_escape_string($conn,$_POST['product_id']);
    $quantity=mysqli_real_escape_string($conn,$_POST['quantity']);
    $productData=getSingleProduct("SELECT id,name,
                                            quantity as quantity_in_stock,
                                            original_price,discounted_price 
                                    from `products` 
                                    where id=".$productId."");
    $productData['quantity']=$quantity;
    switch ($mode) {
        case 'add-to-cart':
            if(empty($getUserTemp['products'])){
                $getUserTemp['products'][$productId]=$productData;
            }else{
                foreach ($getUserTemp['products'] as $key =>  $value) {
                    
                    if($key == $productId){
                        $value['quantity']+=$quantity;
                        $value[$productId]=$value;
                    }else{
                        $getUserTemp['products'][$productId]=$productData;
                    }
                }
            }
            mysqli_query($conn,"UPDATE `session_cart` set session_data='".json_encode($getUserTemp)."' where user_id='".$currentLoggedUserId."'");
    
               
            print_r($getUserTemp);
             break;
    }

My array data:

Array
(
    [user] => Array
        (
            [id] => 8
            [profile_id] => 6
            [user_name] => [email protected]
            [role] => user
            [password] => $2y$10$bL4LybXcOX6nkEgRAM6Jjerrz2czEZJLWv7W1MNg.vWscb39NEsx2
        )

    [products] => Array
        (
            [38] => Array
                (
                    [id] => 38
                    [name] => HP
                    [quantity_in_stock] => 33
                    [original_price] => 50.00
                    [discounted_price] => 40.00
                    [quantity] => 1
                )
            [8] => Array
                (
                    [id] => 8
                    [name] => ASUS VivoBook 15 (2021) Core i3 10th Gen - (8 GB/512 GB SSD/Windows 11 Home) X515JA-EJ362WS Thin and Light Laptop  (15.6 inch, Transparent Silver, 1.80 kg, With MS Office)
                    [quantity_in_stock] => 20
                    [original_price] => 50.00
                    [discounted_price] => 40.00
                    [quantity] => 1
                )
        )
)