Setup devshell.

flake.nix

@@ -0,0 +1,17 @@
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs }:
+  let
+    system = "x86_64-linux";
+    pkgs = nixpkgs.legacyPackages.${system};
+  in {
+    devShells.${system}.default = pkgs.mkShell {
+      buildInputs = with pkgs; [
+        php84Packages.composer
+      ];
+    };
+  };
+}