Code without tests is bad code. It doesn't matter how well written it is; it doesn't matter how pretty or object-oriented or well-encapsulated it is. With tests, we can change the behavior of our code quickly and verifiably. Without them, we really don't know if our code is getting better or worse.
-- Michael C. Feathers 'Working Effectively with Legacy Code'

(not using tests) I like to call Edit and Pray
-- Michael C. Feathers 'Working Effectively with Legacy Code'

Even good programmers make mistakes. The difference between a good programmer and a bad programmer is that the good programmer uses tests to detect his mistakes as soon as possible.
-- Sebastian Bergmann 'PHPUnit Pocket Guide'

They (unit test frameworks) are not solely tools for testing; they can also be used as development tools on a par with preprocessors and debuggers.
-- Paul Hamill 'Unit test frameworks'

Recently, unit testing — in particular Test Driven Development — has been associated with agile development methodologies such as Extreme Programming (XP) that focus on rapid iterations of releasing functional code to customers and welcoming changing customer requirements as a natural part of the development process
-- Jason Sweat 'PHP Design Patterns'