The SitePoint PHP blog has an article posted by Vitalij Mik showing you how to merge the concepts of "clean code architecture" and test-driven development to make solid, maintainable code. The ideals of the "clean code architecture" were first proposed by Robert C. Martin in this post on the 8thlight blog.

The idea was to create an architecture which is independent of any external agency. Your business logic should not be coupled to a framework, a database, or to the web itself. [...] Frameworks will continue to change and evolve. With composer, it is easy to install and replace packages, but it is also easy to abandon a package (composer even has the option to mark a package as abandoned), so it is easy to make “the wrong choice”.

In this tutorial, I will show you how we can implement the Clean Code Architecture in PHP, in order to be in control of our own logic, without being dependent on external providers, but while still using them. We will create a simple guestbook application.

He starts with a first test, evaluating that a list of entries for the guestbook is empty. The code shows the "fakes" for the different object types Uncle Bob recommended in his article and how it fails because none of them exist yet. He extends this with a "can see entries" test and then starts in on the "use case" class to start making the test pass. He updates the case to pull in entries from the repository, another external dependency created later in the tutorial. He then goes through creating the classes for the "fakes" in the test and refactoring the test based on some of his changes during their development. In the remainder of the post he talks about the independence of the current setup and how to add in pagination functionality on the entries objects.