php检查列是否存在

Hello stack community,

For my study i need to understand and learn php. So i made a website and now i have a problem that i can't solve. Within the website there is a shop were 'users' can buy ingame currency with "diamonds". How can i let my users buy something else then currency with their diamonds? I tried multiple ways but it keeps adding the value to the currency instead of the other item.

I tried to find something on the web regarding to this that could help but i can't seem to find anything related to this problem.

Here is the original code:

if($item['currency'] == "") 
{
    //if isset get buy currency
    if(isset($_GET['diamonds'])) 
    {
        $id = mysqli_real_escape_string($link, $_GET['diamonds']);
        $databaseW = mysqli_query($link, "SELECT * FROM `shop_diamonds` WHERE `id`='$id'") or die(mysqli_error($link));
        $itemW = mysqli_fetch_assoc($databaseW);            

        if(mysqli_num_rows($databaseW) == 1)
        {
            if($itemW['price'] <= $item['diamonds']) 
            {
                echo("<p>You bought the product!</p>");
                mysqli_query($link, "UPDATE `users` SET `cash`=`cash`+'{$itemW['hoeveelheid']}', `diamonds`=`diamonds`-'{$itemW['prijs']}' WHERE `user`='{$_SESSION['users']}'") or die(mysqli_error($link));
            }
            else 
            {
                echo("<p>You do not have enough diamonds to buy this product</p>");
            }

        }
        else 
        {
            echo("<p>This product does not exist</p>");
        }
    }

I hope someone could help me with this because i am lost. My question is, how can i let my 'users' buy something else then currency with their daimonds?

If anyone needs more information feel free to ask and i will post it as soon as possible.

Thanks to everyone in regards.