So, there's a PR in staging-next that'll fix the core issue of pg_config not returning what the build system expects because the one in postgresql.out is just a dumb wrapper, the actual executable is in postgresql.dev: https://github.com/NixOS/nixpkgs/pull/341421

Now why does it work in php-src-nix then? The nixpkgs input there is too old and didn't contain the changes to the package outputs that caused this regression in PHP (amongst others).
Why does it work on master? because we do lib.getDev postgresql there (i.e. the regression got fixed).
According to nix show-derivation, the pdo_pgsql from nix-shell/php-src-nix still gets the out instead of dev output from postgresql and thus the wrong pg_config.

I think you need to make sure that pdo_pgsql gets the dev ouptut as it's the case in nixpkgs (I thought these projects re-use the package definitions from nixpkgs, so perhaps there's just an outdated nixpkgs bump at fault somewhere?)
tired and unfocused right now, so I can't tell you which of the three nixpkgs inputs in nix-shell is at fault

In reply to @patka:envs.net
Any problems with me slowly converting things to buildComposerProject2?

I've following error phpize: command not found with

{ sources ? import ../npins
, nixpkgs ? sources."nixos-unstable"
, pkgs
, buildPecl ? pkgs.callPackage (nixpkgs + "/pkgs/build-support/php/build-pecl.nix") { }
,
}:

let
  versionData =
    {
      version = "1.2.4";
      sha256 = "sha256-s9W9aZpQsJLdzZ/d2E1iGDsMTAAjeWbOgWeKP6nNp0A=";
    };
in
buildPecl
{
  pname = "parallel";
  inherit (versionData) version sha256;
}