SESSION Cart adding 2 + or -2 problem - php

Question

So i edited my own shop but im having some issues with it, for example it add 2 instead of 1 or it removes 2 instead of 1,
you can see how it looks on www.neobotmx.org/test/tienda.php <<< not opwn for the public yet >> thats why its on a test folder
The shop code :
<?php
$product_id = $_GET[id]; //the product id from the URL
$action = $_GET[action]; //the action from the URL
//if there is an product_id and that product_id doesn't exist display an error message
if($product_id && !productExists($product_id)) {
die("Error. Product Doesn't Exist");
}
switch($action) { //decide what to do
case "add":
$_SESSION['cart'][$product_id]++; //add one to the quantity of the product with id $product_id
break;
case "remove":
$_SESSION['cart'][$product_id]--; //remove one from the quantity of the product with id $product_id
if($_SESSION['cart'][$product_id] == 0) unset($_SESSION['cart'][$product_id]); //if the quantity is zero, remove it completely (using the 'unset' function) - otherwise is will show zero, then -1, -2 etc when the user keeps removing items.
break;
case "empty":
unset($_SESSION['cart']); //unset the whole cart, i.e. empty the cart.
break;
}
?>
<?php
if($_SESSION['cart']) { //if the cart isn't empty
//show the cart
echo "<table border=\"1\" align=\"center\" padding=\"3\" width=\"70%\">";
echo "<tr>";
//show this information in table cells
echo "<td align=\"center\"><strong>Producto</strong></td>";
//along with a 'remove' link next to the quantity - which links to this page, but with an action of remove, and the id of the current product
echo "<td align=\"center\"><strong>Cantidad</strong></td>";
echo "<td align=\"center\"><strong>Costo</strong></td>";
echo "</tr>";//format the cart using a HTML table
//iterate through the cart, the $product_id is the key and $quantity is the value
foreach($_SESSION['cart'] as $product_id => $quantity) {
//get the name, description and price from the database - this will depend on your database implementation.
//use sprintf to make sure that $product_id is inserted into the query as a number - to prevent SQL injection
$sql = sprintf("SELECT name, description, price FROM products WHERE id = %d;",
$product_id);
$result = mysql_query($sql);
//Only display the row if there is a product (though there should always be as we have already checked)
if(mysql_num_rows($result) > 0) {
list($name, $description, $price) = mysql_fetch_row($result);
$line_cost = $price * $quantity; //work out the line cost
$total = $total + $line_cost; //add to the total cost
echo "<tr>";
//show this information in table cells
echo "<td align=\"center\"><strong>$name</strong></td>";
//along with a 'remove' link next to the quantity - which links to this page, but with an action of remove, and the id of the current product
echo "<td align=\"center\"><strong>$quantity </strong>Borrar</td>";
echo "<td align=\"center\"><strong>$line_cost</strong></td>";
echo "</tr>";
}
}
//show the total
echo "<tr>";
echo "<td colspan=\"2\" align=\"right\"><strong>Total</strong></td>";
echo "<td align=\"right\"><strong>$total</strong></td>";
echo "</tr>";
echo "</table>";
}else{
//otherwise tell the user they have no items in their cart
echo "No tiene articulos en compra.";
}
//function to check if a product exists
function productExists($product_id) {
//use sprintf to make sure that $product_id is inserted into the query as a number - to prevent SQL injection
$sql = sprintf("SELECT * FROM products WHERE id = %d;",
$product_id);
return mysql_num_rows(mysql_query($sql)) > 0;
}
?>
</p>
<p><strong>Seguir Comprando</strong></p>
<?php
and now the display of the books / items / whatever you want.
<?php
define('MAX_REC_PER_PAGE', 1);
$sql = "SELECT id, name, description, price FROM products;";
$rs = mysql_query("SELECT COUNT(*) FROM products") or die("Imposible Realizar Operacion");
list($total) = mysql_fetch_row($rs);
$total_pages = ceil($total / MAX_REC_PER_PAGE);
$page = intval(#$_GET["page"]);
if (0 == $page){
$page = 1;
}
$start = MAX_REC_PER_PAGE * ($page - 1);
$max = MAX_REC_PER_PAGE;
$rs = mysql_query("SELECT id, name, description, price FROM products ORDER BY id
ASC LIMIT $start, $max") or die("Imposible Realizar Operacion");
?>
<table width="100%" height="404" border="0" cellpadding="12">
<?php
while (list($id, $name, $description, $price) = mysql_fetch_row($rs)) {
?>
<tr>
<td height="46" align="left" valign="middle"><p><strong> Producto :
<?= htmlspecialchars($name) ?>
</strong>
</p></td>
</tr>
<tr>
<td height="172" align="left" valign="middle"><p><strong>Descripcion :</strong></p>
<p>
<strong>
<?= htmlspecialchars($description) ?>
</strong></p></td>
</tr>
<tr>
<td height="67" align="left" valign="middle"><p><strong>Precio :
<?= htmlspecialchars($price) ?> </strong>
</p></td>
</tr>
<tr>
<td height="109" align="center" valign="middle"><strong><? echo "Comprar" ?> </strong></td>
</tr>
<?php
}
?>
</table>
<table border="0" cellpadding="5" align="center">
<tr>
<td><strong>Pagina : </strong></td>
<?php
for ($i = 1; $i <= $total_pages; $i++) {
$txt = $i;
if ($page != $i)
$txt = "$txt";
?>
<td align="center"><?= $txt ?></td>
<?php
}
?>
</table>
I have no idea where's the error on it...
Ty for the help :)
Obiusly you have to :
<?php session_start();?>
include your database
etc

Answer 1

You have in the style:
body {
background-image: url();
}
which is causing the browser to request the page again, which adds it to the cart again.
Instead of rendering the cart page, Once the code has modified the cart it should send a redirect to the cart page.