Php输出不正确

Question

<?php

$name = $_POST['name'];
$brand = $_POST['brand'];
$size = $_POST['size'];
$quantity = $_POST['quantity'];
$totalamount = 0.00;


$con=mysqli_connect('localhost', 'username', 'password', 'database');   
// Check connection
if (mysqli_connect_errno())
  {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
  }

$result = mysqli_query($con,"SELECT name,quantity,size,price FROM database.store 
WHERE name='$name' and manufacturer='$brand' and quantity='$quantity' and size='$size';");
$row = mysqli_fetch_array ($result);

    $totalamount=$quantity * $row['price'];
    echo "<br />Your subtotal is: $".number_format($totalamount,2)."<br />";
    mysqli_query($con,"UPDATE  store SET quantity=quantity-'$quantity' WHERE name='$name' AND size='$size'");

mysqli_close($con);
?>

此php的目标是允许用户购买商品。当用户购买东西时，数据库会相应更新。

我现在的问题是，输出没有显示正确的小计，它只显示：“你的小计是：$ 0.00”。但我的数据库正确更新了用户购买的商品数量。