Products not being added to cart - php

Question

Am having issues with my cart script, i've managed to get it to work by adding an item to the cart, but its not allowing any more items to be added but rather it allows the quantity to be added even when i add another item, its the quantity of the first item that will be increased, here is my code snippet
This my function script that wrietes and generates the cart
<?php
function writeShoppingCart() {
$cart = $_SESSION['cart'];
if (!$cart) {
return '<p>You have no items in your shopping cart</p>';
} else {
// Parse the cart session variable
$items = explode(',',$cart);
$s = (count($items) > 1) ? 's':'';
return '<p>You have '.count($items).' item'.$s.' in your shopping cart</p>';
}
}
function showCart() {
$currency = 'UGX';
global $db;
$cart = $_SESSION['cart'];
if ($cart) {
$items = explode(',',$cart);
$contents = array();
foreach ($items as $item) {
$contents[$item] = (isset($contents[$item])) ? $contents[$item] + 1 : 1;
}
$output[] = '<form action="cart.php?action=update" method="post" id="cart">';
$output[] = '<table class="table table-striped">';
foreach ($contents as $id=>$qty) {
$sql = 'SELECT * FROM products WHERE product_id = '.$id;
$result = $db->query($sql);
$row = $result->fetch();
extract($row);
$output[] = '<tr>';
$output[] = '<td>Remove</td>';
$output[] = '<td>'.$product_name.'</td>';
$output[] = '<td>'.$currency.''.$product_price.'</td>';
$output[] = '<td><input type="text" name="qty'.$id.'" value="'.$qty.'" size="3" maxlength="3" /></td>';
$output[] = '<td>'.$currency.''.($product_price * $qty).'</td>';
$total += $product_price * $qty;
$output[] = '</tr>';
}
$output[] = '</table>';
$output[] = '<p>Grand total: <strong>'.$currency.''.$total.'</strong></p>';
$output[] = '<div><button type="submit">Update cart</button></div>';
$output[] = '</form>';
} else {
$output[] = '<p>You shopping cart is empty.</p>';
}
return join('',$output);
}
?>
This is my cart script that handles actions from the cart buttons(add, delete and update)
<?php
session_start();
$cart = $_SESSION['cart'];
$action = $_GET['action'];
switch ($action) {
case 'add':
if ($cart) {
$cart .= ','.!isset($_GET['product_id']);
} else {
$cart = !isset($_GET['product_id']);
}
break;
case 'delete':
if ($cart) {
$items = explode(',',$cart);
$newcart = '';
foreach ($items as $item) {
if (!isset($_GET['product_id']) != $item) {
if ($newcart != '') {
$newcart .= ','.$item;
} else {
$newcart = $item;
}
}
}
$cart = $newcart;
}
break;
case 'update':
if ($cart) {
$newcart = '';
foreach ($_POST as $key=>$value) {
if (stristr($key,'qty')) {
$id = str_replace('qty','',$key);
$items = ($newcart != '') ? explode(',',$newcart) : explode(',',$cart);
$newcart = '';
foreach ($items as $item) {
if ($id != $item) {
if ($newcart != '') {
$newcart .= ','.$item;
} else {
$newcart = $item;
}
}
}
for ($i=1;$i<=$value;$i++) {
if ($newcart != '') {
$newcart .= ','.$id;
} else {
$newcart = $id;
}
}
}
}
}
$cart = $newcart;
break;
}
$_SESSION['cart'] = $cart;
?>
And this is how send actions to my cart from the add button
<form action="cart.php?action=add&id=<?php echo $row_prdt_details['product_id']; ?>" method="post" id="cart">
<fieldset>
<address>
<strong>Name:</strong> <span><?php echo $row_prdt_details['product_name']; ?></span><br>
<strong>Cake Code:</strong> <span><?php echo $row_prdt_details['product_id']; ?></span><br>
<strong>Availability:</strong> <span>
</span><br>
</address>
<h4><strong>Price: UGX <?php echo $row_prdt_details['product_price']; ?></strong></h4>
<p> </p>
<label>Qty:</label>
<input type="text" class="span1" name="qty" placeholder="1">
<input type="submit" onClick="window.location.reload()" />
</fieldset>
</form>
All i want to achieve is to be able to add more items
Any help is greatly welcomed, if u have any questions about what am doing please ask me