I design for PHP5 now exclusively, having dropped development of PHP4 compatible software. PHP5 is so much closer to the model I'm used to in other languages it takes less mental "switch time" to get thinking in a different language.
__________________ Programmers are in a race with the Universe to create bigger and better idiot-proof programs, while the Universe is trying to create bigger and better idiots. So far the Universe is winning. - Rich Cook
I've been coding in PHP 5 for well over a year now. There's far too much in PHP 5 to still stick with PHP 4. Of course, unless the client requests it. But it's just so difficult to remember what wasn't supported in PHP 4 - results in frequent visits to PHP.net.
The man who comes back through the Door in the Wall will never be quite the same as the man who went out.
i started on php3. php4 came out just after i started, and now i prefer php5.
But as far as work goes, most of the things I write have to be written backwards compliant because so many of the clients are on 4, not 5. And some of the original framework versions are still in PHP3, but run on php4 and 5. And don't even get me started with the headache of MySQL versions...
So yeah... I'd love to say I'm "PHP5 Only!", but hey, businesses can't always move at the speed of technology ;)
And that's why it's always better to deliver solutions on your own web servers. That way, you don't need to write aged code and you can keep up with the coding standards and practices (although ALOT - if not all - of the big players in the web hosting business migrated to PHP 5 since september - october last year).
I have optimistic thoughts, even though sometimes (if not always) life's a bitch.