Having put together a couple websites (Redstone Studios and Althea) that integrate with Paypal for providing checkout capabilities, I was very interested to look at Google Checkout when it was released this past week. My experience with the Paypal integration was not so good – while I eventually got it to work, it took a ton of experimentation, searching for documentation, and guessing why things weren’t working (i.e. no meaningful error messages). Luckily, Google Checkout doesn’t seem to suffer from these problems – I had it up and running in no time and it seems very solid for what it does. What does it do? Well, it’s basically a checkout system where you can pass in a cart, users can pay by credit card, and then the money gets deposited in your bank account. Google charges 2% of the transaction amount plus twenty cents. The two percent covers the percent Google has to pay the credit card companies, and the twenty cents is some extra cash for Google. If you use Google Adwords, you get a free credit on Google Checkout – for every 1 dollar spent on Adwords you get 10 dollars of sales with no costs.

Google provides sample PHP code for getting the integration going, but unfortunately the code is PHP4 only, and it is ugly ugly ugly code. I decided to put together a simple class that allows simple integration with Google Checkout. The class is PHP5 only because it uses PHP5’s DOM extension.

To use Google Checkout you need to create a Google Checkout account. From there, you can get your merchant id and merchant key, both of which are needed to communicate with Google Checkout. The Google Checkout API works by POSTING XML over HTTP. While Google’s API is very flexible, I’ve chosen to implement a subset that I think would be useful for many people. Also, I’ve only implemented this class to support the creation of the cart and submission to Google, I haven’t included any functionality for greater integration. This greater integration would involve creating web services that Google could call to update the website on changes to the order status, shipping, etc. The way I’ve set this up, all of that is skipped, and store owner would receive orders via email or Google’s Checkout interface.


  • No support for cart expiration
  • No support for merchant calculations for tax/shipping/gift certificates as these require HTTPS and I don’t have it and don’t care to use it
  • No support for requesting the buyers phone number because I didn’t notice that field until just now!
  • No support for shipping restrictions – i.e. shipping option 1 is only available in these zip codes/states/etc
  • No support for complicated tax rules. Tax can be setup on a state by state basis. One alternative tax rule ‘taxfree’ is setup to allow certain items to be tax free. No support for other alternative tax rules.
  • Currently doesn’t support customization of the form and checkout image

In order to work around some of these limitations, I’ve provided a method called setDefaultXML which allows the developer to specify a default XML file to be used. The class will then fill in the items, do the encryption, and generate the form and checkout image. This allows much greater flexibility and support for all of Google’s options, when needed.

Here’s a simple example of how I might use it:

$merchant_id = 'XXXXXXXXXXXXXXX';
$google = new Gregphoto_Service_Google_Checkout($merchant_id,$merchant_key);
$google->addItem('Doolittle','Doolitle CD by The Pixies (1989)','11.98',1);
$google->addItem('PSCREEN18','18 inch Pizza Screen','7.99',3);
$google->addItem('MYWIDGET01','New tax free widget - Student Use Only','149.21',1,'','taxfree');
$google->addItem('NDISC01','New Customer Discount','-10',1);
$google->editCartUrl = 'http://www.gregphoto.net/index.php?action=editcart&source=google';
$google->continueShoppingUrl = 'http://www.gregphoto.net';
$google->setShipping('UPS Ground',19.99);
$google->setShipping('UPS Next Day',27.99);
$google->setShipping('Instore Pickup',4.99,'pickup');
echo $google->getHTMLForm();

Now, for what’s going on:

  • Line 1 includes the class
  • Lines 3 and 4 setup variables for the merchant id and merchant key
  • Line 6 instantiates the class, passing in the merchant id and key
  • Line 7 sets the class to use the Google Checkout Sandbox – this is a testing sandbox where you can setup fake accounts to use while creating your site. Paypal offerers a much richer sandbox functionality.
  • Lines 8-11 add new line items to the cart, setting there name, description, price, quantity, notes, and special tax rules
  • Lines 12 and 13 setup the URLs the user should be sent to if the choose to edit the cart or continue shopping
  • Lines 14-16 setup various shipping options including their prices
  • Lines 17 and 18 setup taxes for the users in specific states. Google Checkout supports much richer rules around taxation, but I kept it simple on a state-by-state level.
  • Line 20 outputs the HTML form and checkout image that will send the cart to Google Checkout and bring the user to the Google Checkout page
  • Line 21 shows the XML cart, for debugging purposes

View this example running on my site
View the API of Gregphoto_Service_Google_Checkout
Download the class and API docs
View Google’s Develper Guide for Google Checkout