composer.json is wrong

[delboy1978uk]

PHPUnit is in your require-dev section of composer.json, which is usually the sensible place for it. But your code fails on our project, since it only has version 5.7.27 of PHPUnit installed, and Composer ignores your require-dev stuff.

As your actual code extends v6.0 classes, you need to move the phpunit entry from require-dev into require.

[Nyholm]

This is fixed in 4.0. We use composer's conflict section