I’ve been successfully running different versions of PHP alongside eachother, using a forked version of the nixpkgs repo. The reason being I felt more comfortable adjusting the existing repo instead of creating the clever overlays needed to achieve this. Since version 19.03 this is now possible by default thanks to @wmertens and others.
Currently I’m looking to move away from my forked nixpkgs version and want to try overlays, but I’m failing the in-depth knowledge on how to achieve this. PHP extensions (eg. php72Packages.composer) seem not easy to be extended. Can anyone help me with this? How would I overlay / extend / adjust a config to have a custom php extension added, adding a --with-xxx build option to php itself?