I use Zend which isn't free. PDT( free http://www.eclipse.org/pdt/ ) is worth looking into if you have the power to run it. It is also what the next zend is being built on. I have had a look at it and it shows a lot of promise at this point. PHP needs a free environment that can compete professionaly with other languages so it picks up more in high money environments thus Zend contributed a lot to it.
If you plan on writing or using re-usable components ide's are pretty important. There is a limitation what you can engineer imposed by your own memory constaints and the tools chosen to be used(think 1000,10000,100000 lines of code).
Have a play with some MVC pattern combined with a templating engine. If you are really interested in this it is a harder start but gets you fitter faster. If you plan to use other languages it will also come in handy as it is a common working pattern.
Most of the reason you see stuff in the format you do is because it is the easiest to consume for new people. It is the good thing about PHP in that the community is excellant and very hobbyist friendly. Sense of humour is always a plus as well.
PHP is still heavily looked down upon( deemed mostly to be a hobbyist thing ) but with PHP5 is is getting a lot more respect.