I inherited a fairly large, homemade, php4+MySQL, ecommerce project from developers that literally taught themselves programming and html as they wrote it. (I would shudder except that it's really impressive that they were able to do so much by starting from scratch.) My job is to maintain it and take it forward with new functionality.
Functionality of the code depends on $_SESSION data and other global state structures, which then affect the flow of the code and which parts of the site are displayed through require statements. When I took it over last year, my first task was abstracting all of the repetition into separate files which get included via require statements and also removing most of the 'logic' code from the 'display' or output code, but I couldn't remove it all. I have moved code into functions where I can, but that's still quite limited. Classes and methods are definitely out of the question right now.
All testing is being done manually/visually. I would like to start automating some testing, but I simply don't know where to start. Unit testing of functions is pretty straightforward, but very little of the code is in functions and most of that is pretty simple. I've looked at phpUnit and DbUnit, but all of the examples and discussion about them focus on classes and methods.
So, what options do I have to begin implementing unit testing on anything more than the most trivial parts of my project?
I would probably start testing with a tool like Watir or Selenium. This will allow you to do black box testing of the the whole page automatically. Once you have set up these tests, you can start to refactor the PHP pages, and build up unit tests as you are doing the refactoring.