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 = pkgs.php74.buildEnv {
        extensions = ({ enabled, all }: enabled ++ (with all; [
          redis
          xdebug
        ]));
        extraConfig = ''
          xdebug.mode=debug
          date.timezone=Australia/Brisbane
        '';
      };
    in
    [
      php74
      custom.phpPackages.composer
      custom.php74Extensions.pdo
      custom.php74Extensions.redis
      custom.php74Extensions.xdebug
    ];
}

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/0e4ade244d0f1b01a378aa58c235a1576778c1fd) {};
      php74 = pkgs.php.buildEnv {
        extensions = ({ enabled, all }: enabled ++ (with all; [
          redis
          xdebug
        ]));
        extraConfig = ''
          xdebug.mode=debug
          date.timezone=Australia/Brisbane
        '';
      };
    in
    [
      php74
      custom.phpPackages.composer
      custom.php74Extensions.pdo
      custom.php74Extensions.redis
      custom.php74Extensions.xdebug
    ];
}

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
    ];
}

In reply to @rikudou:lemmings.world
Also, you might want to use a project of mine for getting older versions, for example: https://history.nix-packages.com/search?packageName=php74

It should have the latest version the package is found in, meaning the newest available dependencies for each version

In reply to @drupol:matrix.org
You should add the template for flake inputs ! :)

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.