php: fix the filter tests that break the 8.5.12 build

The 8.5.12 pin in 39ed0cc/be9afb2 builds the interpreter but the system build
stopped in php-filter's checkPhase:

  FAILED TEST SUMMARY
  filter.default=encoded doesn't reach hard limit  [tests/oom_filter_default_encoded.phpt]
  filter_input() doesn't reach hard limit          [tests/oom_filter_input.phpt]
  filter_input_array() doesn't reach hard limit    [tests/oom_filter_input_array.phpt]
  make: *** [Makefile:133: test] Error 1

Not a fault in the pin. nixpkgs configures PHP with `--disable-all` and builds
every extension as a shared module, while these three tests — added by the
upstream fix for CVE-2026-109618 — deliberately spawn `php-cgi -n` with no
php.ini.  With no ini there is no shared filter module, so filter_input() is
undefined, the request never reaches the code under test, and the expected
"Allowed memory size ... exhausted" never appears.

nixpkgs shipped the remedy with the version bump in a5fc1be5b6 ("php: fix
`filter` tests", 2026-10-21 14:27 UTC), which adds the module to the child's
command line.  The pinned nixpkgs revision (7694c4f6b853, 2026-10-16) predates
it, so apply the same substitution locally.

The override has to rewrite the filter derivation inside `extensions` rather
than go through buildEnv's `packageOverrides`: that argument is threaded into
the unwrapped PHP only, while the extension set is folded from mkPhp's own
packageOverrides, so passing it there is accepted and silently ignored.

Drop this block once flake.lock is past a5fc1be5b6 — otherwise both patch the
same files and the substitution is applied twice.

Verified by evaluating both derivations against the pinned revision: the
filter derivation now carries the substitution, php-with-extensions is
8.5.12, and the 45-extension set is unchanged.

modules/php.nix

@@ -18,9 +18,42 @@
 #
 # To fall back to PHP 8.4 (nixpkgs' current default `pkgs.php`) change only
 # the `phpBase` line below.
+#
+# Security floor (October 2026):
+#   CVE-2026-109618 (RCE via OOM corruption) and CVE-2026-109619 (RCE via
+#   stale global state) are critical, unauthenticated flaws in PHP itself,
+#   CVSS v4.0 9.2 each, fixed in 8.5.12 / 8.4.27 / 8.3.36 / 8.2.35 on
+#   2026-10-21.  PHP-FPM is named as susceptible in both advisories, so the
+#   pools below are in scope; updating is the only complete mitigation.
+#   nixpkgs bumped php85 in 81513f364091c759812f716cef3c175109d07342 the same
+#   day, but the nixpkgs revision this flake pins predates it, so the floor is
+#   enforced here.  Once flake.lock is at or past that commit the `else`
+#   branch below stops being taken and can be deleted.
 
 let
-	phpBase = pkgs.php85;
+	# The first 8.5.x release that carries the fixes above.  Bump only upward.
+	phpSecurityFloor = "8.5.12";
+
+	# php-8.5.12.tar.bz2, the same artifact and URL shape nixpkgs' own
+	# generic.nix fetches.  Verified against php.net on 2026-10-23.
+	phpSourceHash = "sha256-/7YO3EN87WO1E35JhNs3moERxQBJxxFRDYuVgnfvwag=";
+
+	# `.override` (not `overrideAttrs`) on purpose: php85 is a `makeOverridable`
+	# buildEnv, so this threads `version`/`phpSrc` into generic.nix while
+	# keeping the default extension set.  `overrideAttrs` would keep the new
+	# source but drop the extension set, which would silently strip gd, intl,
+	# pdo_pgsql and friends out of the pools below.
+	phpBase =
+		if lib.versionAtLeast pkgs.php85.version phpSecurityFloor then
+			pkgs.php85
+		else
+			pkgs.php85.override {
+				version = phpSecurityFloor;
+				phpSrc = pkgs.fetchurl {
+					url = "https://www.php.net/distributions/php-${phpSecurityFloor}.tar.bz2";
+					hash = phpSourceHash;
+				};
+			};
 
 	custom-php = phpBase.buildEnv {
 		# `enabled` is nixpkgs' default extension set.  It already contains every
@@ -29,7 +62,41 @@ let
 		# xmlreader, xmlwriter, zip, zlib, pdo_pgsql, pdo_mysql, bcmath, gmp,
 		# exif, sodium, sysvsem, pcntl, ...).  OPcache is compiled into PHP >= 8.5
 		# and no longer appears as a separate extension.
-		extensions = { enabled, all }: enabled ++ (with all; [
+		extensions = { enabled, all }:
+			let
+				# nixpkgs builds PHP with `--disable-all` and every extension as a
+				# shared module.  Three of the OOM regression tests shipped with
+				# 8.5.12 spawn `php-cgi -n` on purpose — no php.ini, so no shared
+				# module, so filter_input() is never defined and the request never
+				# reaches the code under test.  php-filter's `make test` then fails
+				# with "doesn't reach hard limit" for oom_filter_input,
+				# oom_filter_input_array and oom_filter_default_encoded, and the
+				# whole system build stops there.
+				#
+				# Upstream nixpkgs fixes this in a5fc1be5b6 ("php: fix `filter`
+				# tests"), shipped alongside the 8.5.12 bump.  This lock predates
+				# it, so apply the same substitution here; drop this once flake.lock
+				# is past a5fc1be5b6, since the two would otherwise both patch.
+				#
+				# Done by rewriting the filter derivation in the extension list
+				# rather than through buildEnv's `packageOverrides`: that argument
+				# only reaches the unwrapped PHP, while the extension set is folded
+				# from mkPhp's own packageOverrides, so it is silently ignored here.
+				patchFilterTests = ext:
+					if (ext.extensionName or "") != "filter" then
+						ext
+					else
+						ext.overrideAttrs (prevAttrs: {
+						postConfigure = (prevAttrs.postConfigure or "") + ''
+							substituteInPlace \
+								tests/oom_filter_input.phpt \
+								tests/oom_filter_input_array.phpt \
+								tests/oom_filter_default_encoded.phpt \
+								--replace-fail "'-n'," "'-n', '-d', 'extension=$PWD/modules/filter.so',"
+						'';
+					});
+			in
+			map patchFilterTests enabled ++ (with all; [
 				bz2        # Nextcloud: bz2 archive support
 				apcu       # Nextcloud: memcache.local  (apc.enable_cli=1 below is mandatory for occ + cron)
 				redis      # Nextcloud: memcache.distributed / file locking once a Redis server is configured