Phil Sturgeon has a post to his site with a good recommendation for those working with APIs and those "magic numbers" that are HTTP status codes - avoid hard coding them in your applications and tests.

A lot of things in programming are argued to death, but one subject where people almost unanimously agree is that magic numbers can be a pain in the ass, and they should be avoided whenever possible. Sadly when it comes to HTTP status codes, people keep on hardcoding them, and it leads to all sorts of confusion. [...] What is 409? If you answer without looking it up on Dash or HTTP Status Dogs then you are a machine.

He shows two implementations of this idea, one in Ruby and the other in Symfony, where the status code value is represented by a constant rather than by a number. The constant correlates to the HTTP status code (number) but the constant makes it easier to read and understand the code. He points out two libraries that can be substituted into your current testing to replace those hard coded values with more expressive versions: lukasoppermann/http-status and Teapot.

Sebastian Bergmann has a new post to his blog (part of a series on testing techniques for testing that difficult code) about the hard-coded dependencies required by your code and how to stub them for easier testing.

A mock object can be used anywhere in the program where the program expects an object of the mocked class. However, this only works as long as the object can be passed into the context where the original object is used.

Ideally this wouldn't be a problem - handled correctly, dependency injection would make it a non-issue. But, because it has been known to happen, PHPUnit gives you the ability, via the set_new_overload method, to capture that object definition and mock it with a reference to another method in the test class.