In reply to @drupol:matrix.org
tgerbet: Are you ok with this? ^^ Eval is red, but that's because of the warning. How to deal with that in nixpkgs?

In reply to @tgerbet:matrix.org
I do not think lib.warnIf can be used for that. It will always be flagged by the nix-instantiate call done by OfBorg and it is not really an issue within nixpkgs code itself but from upstreams.

 engelsystem-migrate-start[1012]: Exception: Code: 0, Message: Call to undefined function Symfony\Polyfill\Mbstring\iconv_substr(), File: vendor/symfony/polyfill-mbstring/Mbstring.php:660, Previous: None, Trace: [{"file":"\/nix\/store\/03xgnc2nzf5z0112j4b1kmm5qpyrapqf-engelsystem-3.5.0\/share\/engelsystem\/vendor\/symfony\/polyf]

Something like this should do the trick

diff --git a/nixos/modules/services/web-apps/engelsystem.nix b/nixos/modules/services/web-apps/engelsystem.nix
index 669620debce5..7fb912415643 100644
--- a/nixos/modules/services/web-apps/engelsystem.nix
+++ b/nixos/modules/services/web-apps/engelsystem.nix
@@ -3,6 +3,8 @@
 let
   inherit (lib) mkDefault mkEnableOption mkIf mkOption types mkPackageOption;
   cfg = config.services.engelsystem;
+  phpExt = pkgs.php.withExtensions
+    ({ enabled, all }: with all; [ filter mysqlnd mysqli pdo pdo_mysql mbstring ] ++ enabled);
 in {
   options = {
     services.engelsystem = {
@@ -99,7 +101,7 @@ in {
       '';
 
     services.phpfpm.pools.engelsystem = {
-      phpPackage = pkgs.php81;
+      phpPackage = phpExt;
       user = "engelsystem";
       settings = {
         "listen.owner" = config.services.nginx.user;
diff --git a/pkgs/servers/web-apps/engelsystem/default.nix b/pkgs/servers/web-apps/engelsystem/default.nix
index 9b9cf0ff3bfe..8274f3e88418 100644
--- a/pkgs/servers/web-apps/engelsystem/default.nix
+++ b/pkgs/servers/web-apps/engelsystem/default.nix
@@ -2,15 +2,15 @@
 
 let
   phpExt = php.withExtensions
-    ({ enabled, all }: with all; [ filter mysqlnd mysqli pdo pdo_mysql ]);
+    ({ enabled, all }: with all; [ filter mysqlnd mysqli pdo pdo_mysql mbstring ] ++ enabled);
 in
 stdenv.mkDerivation rec {
   pname = "engelsystem";
-  version = "3.4.1";
+  version = "3.5.0";
 
   src = fetchzip {
     url = "https://github.com/engelsystem/engelsystem/releases/download/v${version}/engelsystem-v${version}.zip";
-    hash = "sha256-5KniP1nrLfmWHruXnUJmlvgL95U+EsDmCs4tg/YLWtw=";
+    hash = "sha256-RbzAHBZN02u14WaLtq5EOh4XwIdHKvzX7NhDBhn/CaU=";
   };
 
   buildInputs = [ phpExt ];
diff --git a/pkgs/top-level/all-packages.nix b/pkgs/top-level/all-packages.nix
index 1f6edde48394..acd7f66f5b61 100644
--- a/pkgs/top-level/all-packages.nix
+++ b/pkgs/top-level/all-packages.nix
@@ -25999,7 +25999,7 @@ with pkgs;
 
   dspam = callPackage ../servers/mail/dspam { };
 
-  engelsystem = callPackage ../servers/web-apps/engelsystem { php = php81; };
+  engelsystem = callPackage ../servers/web-apps/engelsystem { };
 
   envoy = callPackage ../servers/http/envoy {
     go = go_1_20;

-  engelsystem = callPackage ../servers/web-apps/engelsystem { php = php81; };
+  engelsystem = callPackage ../servers/web-apps/engelsystem { };

Not really, it is still the "usual" approach when it is expected that most of the packages will work fine with the default values (or will at some point in the future)

But yes both work