[mephisto]

Your error is because you are declaring $db inside the AddItem function in kelly.php, but not in UpdateItem or RemoveItem. Personally I'd use a class.

PHP Code:

<?php
    class Kelly {
        private $db;
        public function __construct() {
            $this->db = new mysqli('localhost', 'root', '', 'test');
            $this->db->select_db('test');
        }
        
        public function AddItem($itemID, $qty) {
            $query = "select count(*) from cart where cookieId = '" . GetCartId() . "' and itemId = $itemId";
            $result = $this->db->query($query);
            $row = $result->fetch_assoc();
            $numRows = $row[0];
            if($numRows == 0) {

                // This item doesn't exist in the users cart,
                // we will add it with an insert query

                $query="insert into cart(cookieId, itemId, qty) values('" . GetCartId() . "', $itemId, $qty)";
                $result=$this->db->query($query);
            }
            else {
                // This item already exists in the users cart,
                // we will update it instead

                $this->UpdateItem($itemId, $qty);
            }
        }

        public function UpdateItem($itemID, $qty) {
            $query="update cart set qty = $qty where cookieId = '" . GetCartId() . "' and itemId = $itemId";
            $result=$this->db->query($query);
        }

        public function RemoveItem($itemId){
            $query="delete from cart where cookieId = '" . GetCartId() . "' and itemId = $itemId";
            $result=$this->db->query($query);
        }


        public function ShowCart(){
            $query="select * from cart inner join items on cart.itemId = items.itemId where cart.cookieId = '" . GetCartId() . "' order by items.itemName asc";
            $result=$this->db->query($query);
            $output = "";
            while($row = $result->fetch_assoc()) {
                // Increment the total cost of all items
                $totalCost += ($row["qty"] * $row["itemPrice"]);
                $itemName = $row["itemName"];
                $itemID = $row["itemId"];
                $formatedPrice = number_format($row["itemPrice"], 2, ".", ",");
                $output. = <<< EOT
<td width="55%" height="25">
<font face="verdana" size="1" color="black">
{$itemName}
</font>
</td>
<td width="20%" height="25">
<font face="verdana" size="1" color="black">
{$formatedPrice}
</font>
</td>
<td width="10%" height="25">
<font face="verdana" size="1" color="black">
<a href="cart.php?action=remove_item&id={$itemID}">Remove</a>
</font>
</td>
</tr>
EOT;
            }
            
            $formatedTotal = number_format($totalCost, 2, ".", ",");
            
            $output .= <<< EOT
<tr>
<td width="100%" colspan="4">
<hr size="1" color="red" NOSHADE>
</td>
</tr>
<tr>
<td width="70%" colspan="2">
<font face="verdana" size="1" color="black">
<a href="products.php"><< Keep Shopping</a>
</font>
</td>
<td width="30%" colspan="2">
<font face="verdana" size="2" color="black">
<b>Total: ${$formatedTotal}</b>
</font>
</td>
</tr>
EOT;
        }
    }
?>

Note, all I have done here is took your functions and put them in a class, so that they will use the same database connection. There are still plenty of improvments that could be made, but that's another issue.