It happens all the time while working on a big project. You fix something and something else breaks. Not because you build it as a bad piece of software, but because it is just so damn complex with dependencies everywhere.
Continuous Integration tests should catch these breaks in your application, but they are quite hard to setup. The article below goes further into the different components you can use for CI tests for PHP and even helps to set them up. I found it very helpful to read.
In simple terms, you verify that your project meets the quality standards frequently. This way, you can catch any deviation early. Doing the integration in small increments makes it easier. Implementing a continuous integration server can look difficult when you’ve never done it. In this post I will try to document how to set it up. I take for granted that you already have a LAMP server configured.
/via Continuous Integration In PHP | Eric Hogues Blog.