Common PHP-FPM for example allows for persistent PHP processes to handle multiple requests, reducing the overhead of creating a new instance for each request.
Also then there was no stable JIT while doing this study, so probably situation has changed a lot in a better way, but writing efficient code will always be a requirement.
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
Ranking of the most energy-efficient programming languages (lemmygrad.ml)
2020 study pdf....
No way this can't be true💀 (lemmygrad.ml)