This is a simple example of unit testing in PHP with PHPUnit. The talk has been written up as a blog post: PHPUnit: What, Why, How?.
Please feel free to download this repository and give the unit tests a try.
As mentioned in my blog post try adding a second test case for the Average::median method to test when an even set of numbers is passed to the method.
If you succeed at that, then try taking a test driven development approach to adding a new method to Average for calculating the modal average by writing a test first.
Clone this repository and then run Composer:-
composer install
You will then be able to run the unit tests using:-
./vendor/bin/phpunit