php = pkgs.api.buildPhpFromComposer {
  src = inputs.self;
  php = pkgs.php83; # Change to php81, php82, php83 etc.
  withExtensions = [
    (pkgs.php83.extensions.xdebug.overrideAttrs (oldAttrs: {
      src = pkgs.fetchFromGitHub {
        owner = "xdebug";
        repo = "xdebug";
        rev = "7d10e631c17694913eadafb9a07da22fb3736b2c";
        hash = "sha256-El6kdI1rOTWbwmX8bL1yB9KPwpew8GNK+lOb6h3KiOI=";
      };
    }))];
        };

php = pkgs.api.buildPhpFromComposer {
  src = inputs.self;
  php = pkgs.php83; # Change to php81, php82, php83 etc.
  withExtensions = [
    (pkgs.php83.extensions.xdebug.overrideAttrs (oldAttrs: {
      src = pkgs.fetchFromGitHub {
        owner = "xdebug";
        repo = "xdebug";
        rev = "7d10e631c17694913eadafb9a07da22fb3736b2c";
        hash = "sha256-El6kdI1rOTWbwmX8bL1yB9KPwpew8GNK+lOb6h3KiOI=";
      };
    }))
  ];
};

Seems to be still picking up the stock version. This is the flake: https://github.com/lstrojny/uffff/blob/php-8.3/flake.nix

trace: warning: The pcre extension does not exist or is enabled by default in PHP 8.3.0RC1, ignoring.
trace: warning: The pcre extension does not exist or is enabled by default in PHP 8.3.0RC1, ignoring.
error: builder for '/nix/store/jvaykbfdbmsn32ws08wrym63b490alk6-php-xdebug-3.2.2.drv' failed with exit code 1;
       last 10 log lines:
       > checking for PHP includes... -I/nix/store/h74ww6409cjw2rrvzy9mc5w6cy54imb0-php-8.3.0RC1-dev/include/php -I/nix/store/h74ww6409cjw2rrvzy9mc5w6cy54imb0-php-8.3.0RC1-dev/include/php/main -I/nix/store/h74ww6409cjw2rrvzy9mc5w6cy54imb0-php-8.3.0RC1-dev/include/php/TSRM -I/nix/store/h74ww6409cjw2rrvzy9mc5w6cy54imb0-php-8.3.0RC1-dev/include/php/Zend -I/nix/store/h74ww6409cjw2rrvzy9mc5w6cy54imb0-php-8.3.0RC1-dev/include/php/ext -I/nix/store/h74ww6409cjw2rrvzy9mc5w6cy54imb0-php-8.3.0RC1-dev/include/php/ext/date/lib
       > checking for PHP extension directory... /nix/store/ry064mmz1j95pvnyy8g2f2x8n76yvmsj-php-8.3.0RC1/lib/php/extensions
       > checking for PHP installed headers prefix... /nix/store/h74ww6409cjw2rrvzy9mc5w6cy54imb0-php-8.3.0RC1-dev/include/php
       > checking if debug is enabled... no
       > checking if zts is enabled... no
       > checking for gawk... gawk
       > checking whether to enable Xdebug support... yes, shared
       > checking whether to enable Xdebug developer build flags... no
       > checking whether to compress profiler files (requires zlib)... yes
       > checking Check for supported PHP versions... configure: error: not supported. Need a PHP version >= 8.0.0 and < 8.3.0 (found 8.3.0RC1)
       For full logs, run 'nix log /nix/store/jvaykbfdbmsn32ws08wrym63b490alk6-php-xdebug-3.2.2.drv'.
error: 1 dependencies of derivation '/nix/store/q23pzqnz65dhx33kp8sp8df2n6c5srj5-php-extra-init-8.3.0RC1.ini.drv' failed to build
error: 1 dependencies of derivation '/nix/store/vhx6kp081lwdckaaycs8gcrgfm05gd3r-php-with-extensions-8.3.0RC1.drv' failed to build
error: 1 dependencies of derivation '/nix/store/qy81pchx64hx5bwb84fgk6rmk320gwh7-nix-shell-env.drv' failed to build```

In reply to @drupol:matrix.org
Should we add a package depending on PHP on the top level of nixpkgs or nested under php.packages ? See my comment: https://github.com/NixOS/nixpkgs/pull/252778