带有会话变量的SQL UPDATE

Question

So say that im getting a value from my database and put it into a session variable like below.

$getsteamcoins = $db->prepare("SELECT coins FROM User WHERE steamid=?");
$getsteamcoins->bind_param('i',$steamid);

$getsteamcoins->execute();
$hej2 = $getsteamcoins->get_result();
$hej3 = $hej2->fetch_object();
$_SESSION['steamcoins'] = $hej3->coins;
$getsteamcoins->close();

Then updating the session variable and wanting to update my database with the updated session variable in place of the old one. How do I do that, the problem seem to be that the session variable changes when the page is refreshed, but when I refresh the page the session variable gets the value that is in the database (the old value). Anyway to work around this, help is appriciated :D

$_SESSION['steamcoins'] = $_SESSION['steamcoins'] - $coinsCoinFlip;
    $insertvalues2 = $db->prepare("UPDATE `User` SET `coins`=? WHERE steamid=?");
    $insertvalues2->bind_param("ii",$_SESSION['steamcoins'], $steamid);
    $insertvalues2->execute();
    $insertvalues2->close();