After seeing this a couple years ago I started to be more responsible for my PHP code, writing it even with the slightest micro-optimizations possible, since more times people use my code (especially due to nature of PHP being executed in every http request), more energicity they waste because of my lazyness and more they pollute the planet. JIT was NOT included in this experiment, so the number can be much more lower.
I hope you will be responsible for your code so on too (generally I mean people using interpreted languages).
Yes, you are completely right, if your code writing style is basic, all of it being solved by your application design. Not sure for open files (in contrast with disk cache of most OS), we have configurable persistent db connections, app configurations in most cases being saved in some cache (PHPBB cache is an example). Most of the time the code you wrote is the main factor of your code speed and waste coefficient. Luckily every year we get more modern PHP interpreter, if we look back at PHP 5, it was a complete disaster in the terms of performance, but I don't say that it will be as resource-saving as compiled languages ever (there always will be some cold-start overhead), it's pretty neat programming language, although I now consider lazy PHP developers as planet's nature-killers now :D