How to properly subtract from a database using php and php variables? - php

Question

I am trying to create a shopping cart and take the quantities in the cart to subtract from my inventory in my table in mysql. But everytime i try and update it keeps on subtracting the wrong amount. So far i have this.
<form action="<?php $_SERVER['PHP_SELF'] ?>" method="POST">
<center><B>Invoice</B></center>
<?php
require 'connect.php';
require 'item.php';
if(isset($_GET['id'])){
$result = mysqli_query($con, 'select * from products where
id='.$_GET['id']);
$products = mysqli_fetch_object($result);
$item = new Item();
$item->id = $products->id;
$item->name = $products->name;
$item->price = $products->price;
$item->quantity = 1;
// Check if the products exists in the cart
$index = -1;
$cart = unserialize(serialize($_SESSION['cart']));
for($i=0; $i<count($cart); $i++)
if($cart[$i]->id==$_GET['id'])
{
$index = $i;
break;
}
if($index==-1)
$_SESSION['cart'][] = $item;
else{
$cart[$index]->quantity++;
$_SESSION['cart'] = $cart;
}
}
// Delete products in cart
if(isset($_GET['index'])){
$cart = unserialize(serialize($_SESSION['cart']));
unset($cart[$_GET['index']]);
$cart = array_values($cart);
$_SESSION['cart'] = $cart;
}
?>
<table cellpadding="3" cellspacing="3" border="0">
<tr>
<th>Id</th>
<th>Name</th>
<th>Price</th>
<th>Quantity</th>
<th>Sub-Total</th>
</tr>
<?php
$cart = unserialize(serialize($_SESSION['cart']));
$s = 0;
$index = 0;
for($i=0; $i<count($cart); $i++){
?>
<tr>
<td><?php echo $cart[$i]->id; ?></td>
<td><?php echo $cart[$i]->name; ?></td>
<td><?php echo $cart[$i]->price; ?></td>
<td><?php echo $cart[$i]->quantity; ?></td>
<td><?php echo $cart[$i]->price * $cart[$i]->quantity; ?></td>
</tr>
<?php
$index++;
$quantitycart=$cart[$i]->quantity;
$idcart=$cart[$i]->id;
}
?>
<td><input type="submit" name="submit" value="submit"></td></tr>
<tr><td colspan="5"</tr>
<tr>
</tr>
</table><br>
<?php
if(array_key_exists('submit', $_POST))
{ $results = mysqli_query($con, 'select * from products');
for($i=0; $i<count($cart); $i++)
{
while($products= mysqli_fetch_object($results)){
$quantity= $products->quantity;
$newquantity= $quantity-$quantitycart;
$sql = "UPDATE products SET quantity='$newquantity' WHERE id='$idcart'";
}}
if ($con->query($sql) === TRUE) {
echo "Record updated successfully";
} else {
echo "Error updating record: " . $con->error;
}
$con->close();
}
$con is my connnection. Any help would be appreciated. The main problem is located in the last part of the code.