On the Codeception site they've posted a guide to using the AspectMock feature of the testing tool to prove that "nothing is untestable."

We already announced AspectMock, the mocking framework that may dramatically change the way you do testing in PHP. In this video Jeffrey Way shows how AspectMock is different from others. In this post we will demonstrate its powers too, and we will try to break some stereotypes about PHP testing. To get the code tested, you should always keep in mind how you would write a test for it. We know unit testing requires some good practices to follow and bad practices to avoid.

Their first example involves testing singletons, notorious for being difficult to test because of their "global" state. He also gives a more practical example using a Yii2-based application and a login form. True to its name, the AspectMock uses Aspect Oriented Programming concepts to make the "magic" happen behind the scenes.