Fruga is a static site generator (SSG), written in PHP. It's a free software under GPL licence.
For many websites, static generation can significantly reduce environmental impact.
You can find Fruga here : https://github.com/yoandm/fruga
I've developed it for personal use, but I've also thought of it as generic so that it can be used more widely.
Fruga has been designed to be as simple as possible, avoiding the need for multiple overlays.
The learning curve should therefore be very steep.
You may be surprised by the relatively short documentation.
Documentation to arrive gradually.
You can find the French version here : https://yoandemacedo.com/fruga/doc/fr/
Sorry for the sometimes approximate English. If you want me to correct something, please contact me.