How to calculate sales tax with this script?

Question

I'm using a shopping cart script from a tutorial I came across a while ago, and I need to modify it to calculate sales tax for my state (Pennsylvania). I want it to extract the state, compare it to either Pennsylvania or PA incase someone writes the state shorthand and then multiply the subtotal (for the items.. before shipping is added) by the sales tax for this state and add that to the subtotal. How should I go about doing this? Here is the script that does the calculation for grand total.

<?php
require_once 'config.php';

/*********************************************************
*                 CHECKOUT FUNCTIONS 
*********************************************************/
function saveOrder()
{
    $orderId       = 0;
    $shippingCost  = 5;
    $requiredField = array('hidShippingFirstName', 'hidShippingLastName', 'hidShippingAddress1', 'hidShippingCity', 'hidShippingPostalCode',
                           'hidPaymentFirstName', 'hidPaymentLastName', 'hidPaymentAddress1', 'hidPaymentCity', 'hidPaymentPostalCode');

    if (checkRequiredPost($requiredField)) {
        extract($_POST);

        // make sure the first character in the 
        // customer and city name are properly upper cased
        $hidShippingFirstName = ucwords($hidShippingFirstName);
        $hidShippingLastName  = ucwords($hidShippingLastName);
        $hidPaymentFirstName  = ucwords($hidPaymentFirstName);
        $hidPaymentLastName   = ucwords($hidPaymentLastName);
        $hidShippingCity      = ucwords($hidShippingCity);
        $hidPaymentCity       = ucwords($hidPaymentCity);

        $cartContent = getCartContent();
        $numItem     = count($cartContent);

        // save order & get order id
        $sql = "INSERT INTO tbl_order(od_date, od_last_update, od_shipping_first_name, od_shipping_last_name, od_shipping_address1, 
                                      od_shipping_address2, od_shipping_phone, od_shipping_state, od_shipping_city, od_shipping_postal_code, od_shipping_cost,
                                      od_payment_first_name, od_payment_last_name, od_payment_address1, od_payment_address2, 
                                      od_payment_phone, od_payment_state, od_payment_city, od_payment_postal_code)
                VALUES (NOW(), NOW(), '$hidShippingFirstName', '$hidShippingLastName', '$hidShippingAddress1', 
                        '$hidShippingAddress2', '$hidShippingPhone', '$hidShippingState', '$hidShippingCity', '$hidShippingPostalCode', '$shippingCost',
                        '$hidPaymentFirstName', '$hidPaymentLastName', '$hidPaymentAddress1', 
                        '$hidPaymentAddress2', '$hidPaymentPhone', '$hidPaymentState', '$hidPaymentCity', '$hidPaymentPostalCode')";
        $result = dbQuery($sql);

        // get the order id
        $orderId = dbInsertId();

        if ($orderId) {
            // save order items
            for ($i = 0; $i < $numItem; $i++) {
                $sql = "INSERT INTO tbl_order_item(od_id, pd_id, od_qty)
                        VALUES ($orderId, {$cartContent[$i]['pd_id']}, {$cartContent[$i]['ct_qty']})";
                $result = dbQuery($sql);                    
            }


            // update product stock
            for ($i = 0; $i < $numItem; $i++) {
                $sql = "UPDATE tbl_product 
                        SET pd_qty = pd_qty - {$cartContent[$i]['ct_qty']}
                        WHERE pd_id = {$cartContent[$i]['pd_id']}";
                $result = dbQuery($sql);                    
            }


            // then remove the ordered items from cart
            for ($i = 0; $i < $numItem; $i++) {
                $sql = "DELETE FROM tbl_cart
                        WHERE ct_id = {$cartContent[$i]['ct_id']}";
                $result = dbQuery($sql);                    
            }                           
        }                   
    }

    return $orderId;
}

/*
 Get order total amount ( total purchase + shipping cost )
*/
function getOrderAmount($orderId)
{
    $orderAmount = 0;

    $sql = "SELECT SUM(pd_price * od_qty)
            FROM tbl_order_item oi, tbl_product p 
            WHERE oi.pd_id = p.pd_id and oi.od_id = $orderId

            UNION

            SELECT od_shipping_cost 
            FROM tbl_order
            WHERE od_id = $orderId";
    $result = dbQuery($sql);

    if (dbNumRows($result) == 2) {
        $row = dbFetchRow($result);
        $totalPurchase = $row[0];

        $row = dbFetchRow($result);
        $shippingCost = $row[0];

        $orderAmount = $totalPurchase + $shippingCost;
    }   

    return $orderAmount;    
}

?>

Answer 1

You don't have to handle differing tax rates per municipality? Normally this would be looked up from a tax table by zip code, and maybe even for the street address + zip code (in some states).

It would look like this:

// is it a Pennsylvania zip code?
if ($zipcode < 15000 || $zipcode > 19699)  // needs verification of the actual range
   $taxrate = 0;
else
{
    $rs = mysql_query ("select rate from taxtable where zipcode = '$zipcode'", $db);
    if (!$rs)
    {
        // error looking up rate, maybe apply a default?
    }
    else
    {
        $row = mysql_fetch_assoc($rs);
        $taxrate = $row['rate'];
    }
}

$amount = $amount * (1 + $taxrate / 100.0);