I have a confession to make. I've never really been huge on the testing. At least not automated testing. I haven't been in the biz that long, so it's probably not a huge issue, but in hindsight a think a fair bit of unit testing on previous projects definitely would have helped.
With PHP if you're not using a pre-built framework or something, it might not be so easy to do unit testing. From my limited experience, you need to break things up into small units in order to do unit testing (gee, whoda thunk). While this is usually good development practice, in PHP it is not really enforced. I can think back to some projects in my younger days when things were kinda nasty.
Then you might start implementing some sort of basic unit testing by separating out some common functions and writing tests for those to see if they work best. You might even separate stuff into distinct chunks called actions. Those can be tested fairly well for behaviour, because you can just fire some $_GET or $_POST crap and then check the DB to see if the correct things happened, etc. Bit harder to check the display though, you'll have to dig through a fair bit of outputted HTML to see if things are correct. Unless you're using some sort of template engine, in which case you might have to get some hooks in there to see what is getting passed over.
Jump over to Rails. You've got everything really broken up into bits, there's the controllers that simply spit out raw Ruby objects which are very easily testable. There's the models, which are also very easily testable. Finally they have integration tests, which make sure that if you do a whole bunch of actions in a certain order, the correct result happens and nothing explodes in your face. It might be that each specific piece works fine, but suppose a bunch of actions are going off in succession and somehow one borks the data that a later one wants and everything explodes...this is much easier to catch through integration testing.
So now I've got a Rails app with a whole whack of tests (and believe me, it's not enough) that test a lot of basic functionality. Then the boss comes along and says, "we need this changed! And this and this and this! By tomorrow1!" So I pile on all this functionality and make a big fat thing with code shooting out everywhere and the other coders are crying because there's all this stuff everywhere and they're having trouble following it. The next plan is to sit down and have a good refactor. Clean out some crap that is no longer in use, merge a couple other things that are doing something very similar, blah blah blah. The main problem with refactoring is that stuff breaks. In places you never knew it could possibly happen, like when you work out your arms at the gym and the next day your thigh hurts or something. Fortunately, you've got this wonderful command called "rake test" which then runs all your tests and gives you a nice little error report of everything that died. Then you quickly go through the list, fixing the little things here and there and run the test again. So much faster than this: I-make-change, fix-obvious-bugs-that-I-find, user-finds-bugs, I-track-them-down, I-fix, user-finds-more-bugs, etc.
The more I do this stuff, the more I wonder how I could have possibly coded without all this junk. I guess coming from my static typing world of C++ I had that wonderful thing called the compiler check my code for correctness, but once we're out here in PHP/Ruby-land we don't have such a luxury and need something else to watch our backs (not that unit testing isn't needed in C++, just slightly less). How much time would I have saved had I used more test-driven development from the beginning?
1 This is a bit of an exaggeration, but I'm sure you get the idea.