Update Cash in "sell.php" is Not Working

Question

Everything is working file but the cash is not updating. I think its due to typecasting. The value for shares is an integer and price is a float-point integer but I'm confused as to how to cast them.

Code:

// deletes the user's stock own and update the cash balance.
$stock = lookup($_POST["stocks"]);

// get how much a person own ( cash value of stock ) 
if ( $stock !== false )
{
    $shares = query("SELECT shares FROM stocks WHERE id = ?",$_SESSION["id"] ) ;
    $stockvalue = number_format($shares["shares"],2,'.','')
                                       * number_format($stock["price"], 2, '.', '') ;

    //update the current cash and delete the stocks 
    query("DELETE FROM stocks WHERE id = ?",$_SESSION["id"] );

    query("UPDATE users SET cash = cash + ? WHERE id = ?",$stockvalue, $_SESSION["id"]);

    redirect("index.php");
}