The SitePoint PHP blog has posted a tutorial from author Wern Ancheta digs into the Shopify API and shows you some simple methods to use it with Guzzle.

In this tutorial, you’re going to get started with developing Shopify apps. You’re going to create a simple app that lists out products from a Shopify store. [...] Shopify apps are a way of extending the functionality of a Shopify store or to provide ecommerce capabilities to websites or mobile apps.

The tutorial then starts in, showing you how to set up a Shopify Partner Account and create the "Store" instance you'll be using for the development. With that created, you'll have to set up a new application inside the store - this is what the script will actually connect with. From there they start in on the demo application, installing Twig, Slim, Guzzle and a few other libraries. They show the code to set up the simple Slim application along with a handful of routes, views and some SQL interaction. The tutorial includes the code for:

• authenticating users against the API (and your store)
• making requests to the API for product information
• outputting the results to a simple page

If you're short on time or just want to jump to the end, you can get the code for this example in this GitHub repository.