Your tips to write testable (PHP) code?

I’m going to make a quick seminar for my team on how to improve the testability of our (15 years old) PHP code-base. Do you have any specific tips and tricks we can follow? Currently we’re using the Yii 1.1 framework, with PHPUnit and Facebook WebDriver with Serentiy for functional tests.

Some things I thought about:

• Include dependency injection where possible – e.g. inject the $request object instead of hacking $_POST and $_GET everywhere (but where is it possible in an old code-base?)
• Functions should not hack global state if they don’t need to (related to point above)
• Functions with no side-effects are the easiest functions to test, so side-effects should happen as far up in the stacktrace as possible (hard to implement, examples needed)
• Functions should be short
• Functions should only do one thing
• Functions should have a clear relation between input and output (“contract” in some sense; also related to side-effects)
• First line of a function’s docblock should explain the relation between input and output – this line is what needs to be tested
• Controllers should be as small as possible, and basically just bake the input and feed it to models and helpers
• (Also include example of code that’s hard to test, and how to improve its testability) Thoughts? Which rules or habits do you use in your workplace?

Edit:

Some open questions:

• Are objects more easy to test than arrays? Should associative arrays (always) be replaced by objects?
• What’s best from a testability perspective: using exceptions or result objects/result array to propagate errors?