In reply to @rikudou:lemmings.world
I've updated it a bit, there was a mistake

In reply to @stotes:matrix.org
Just wanted to thank you again for your help. Got me on the right path. First time user of NixOs and even though it's taking me awhile to wrap my head around it I'm loving it so far. Really appreciate you taking the time to help.

In reply to @rikudou:lemmings.world

Here's a fully working shell.nix with what you want, it can easily be adapted to configuration.nix:

{ pkgs ? import <nixpkgs> {} }:
pkgs.mkShell {
    nativeBuildInputs = with pkgs.buildPackages;
    let
      custom = import (builtins.fetchTarball https://github.com/nixos/nixpkgs/tarball/1d8b8365a02efbf668311dc9db06cb98d49e7302) {};
      php74 = custom.php74.buildEnv {
        extensions = ({ enabled, all }: enabled ++ (with all; [
          redis
          xdebug
        ]));
        extraConfig = ''
          xdebug.mode=debug
          date.timezone=Australia/Brisbane
        '';
      };
    in
    [
      php74
      custom.php74Packages.composer
      custom.php74Extensions.pdo
      custom.php74Extensions.redis
      custom.php74Extensions.xdebug
    ];
}