On the CloudWays blog there's a tutorial posted for the CodeIgniter framework users out there showing how to get started with unit testing your application.

Quality assurance is one of the central aspects of software development. In fact, test-driven development is an entire development methodology developed around the concept of integrating quality assurance within the development cycle. However, before discussing how to automate Codeigniter unit testing, I will describe the theoretical basis of unit testing and how it adds value to the Codeigniter projects.

The tutorial starts out by defining what a "unit" is and how testing provides value to your project, making it easier to find issues early on and building in simplicity in its structure. It also talks about some of the limitations of unit testing including the effort involved (and lack or potential gain) and having test code with bugs too. It then starts in on some example tests, showing how to work with configuration objects and built test cases and execute the tests.