I am all for testing. Whether you always write unit tests in advance as Test Driven Development (TDD) advocates call for, write them after, write them alongside, a little of each, I don't care. Tests are your friend. You want tests, and specifically you want good tests.
There's a lot of opinions on what constitutes a "good" test, of course, and much is subjective to the type of code you're working on. However, since the release of PHP 7 I've found that while writing tests... I am never using a mocking library. In fact, I'm going to go as far and say that
You should never use a mocking library in PHP 7.
Before all of you gasp, clutch your pearls, and send ninja hit squads after me, let me justify that position.