Add a PHP 7.4 devshell

modules/php74.nix

@@ -0,0 +1,25 @@
+{ inputs, ... }:
+
+{
+  perSystem =
+    { pkgs, system, ... }:
+    let
+      pkgs-php74 = import inputs.nixpkgs-php74 { inherit system; };
+
+      php = pkgs-php74.php74;
+      phpPackages = pkgs-php74.php74Packages;
+    in
+    {
+      devShells.php74 = pkgs.mkShell {
+        packages = with pkgs; [
+          php
+          phpPackages.composer
+          phpactor
+        ];
+
+        shellHook = ''
+          php --version
+        '';
+      };
+    };
+}