Regression: mb_ucwords() phantom on PROFILE=8.4 — Zend 8.4/8.5 function_exists false (re-#20799, ext/mbstring)

[PurHur]

Category

Regression · php-src-strict · phantom builtin (Pillar 4)

Problem

mb_ucwords() was added as a “PHP 8.4” forward-profile API (#20799 / #21394 / compliance mb_ucwords_forward84.phpt). Zend php-src never ships mb_ucwords — probed 2026-07-20 against php:8.4-cli (8.4.23) and php:8.5-cli (8.5.8) with mbstring enabled: function_exists('mb_ucwords') === false. Title-case on Zend is mb_convert_case($s, MB_CASE_TITLE).

This reverses the wrong-direction “fix” that treated a missing phantom as a real parity gap.

Repro	Zend 8.4.23 / 8.5.8 + mbstring	VM PHP_COMPILER_PROFILE=8.4
function_exists('mb_ucwords')	false	true ❌
mb_ucwords('hello world')	undefined function	Hello World

Default / 8.2 profile already hides it (mb_ucwords_phantom.phpt green).

php-src reference

• php/php-src ext/mbstring/mbstring.c — no mb_ucwords; use mb_convert_case / MB_CASE_TITLE
• Closest Zend API: mb_convert_case() (same file)

PHP implementation target

• Gate off / remove ext/mbstring/mb_ucwords.php registration on all profiles (including 8.4 forward)
• Flip test/compliance/cases/mbstring/mb_ucwords_forward84.phpt (+ related) to expect absence; keep phantom guard
• PHP-in-PHP only; no new C

Repro

./script/docker-exec.sh -- bash -lc 'export PHP_COMPILER_PROFILE=8.4
printf "%s\n" "<?php echo function_exists(\"mb_ucwords\") ? \"exists:yes\" : \"exists:no\";" > /tmp/mb_ucwords_phantom.php
php bin/vm.php /tmp/mb_ucwords_phantom.php
# expect exists:no; currently exists:yes
'

Done when

• PHP_COMPILER_PROFILE=8.4: function_exists('mb_ucwords') === false (match Zend)
• Compliance / unit gates updated (no forward84 “must exist” expectation)
• php-src-strict; no php-compiler-strict shortcut

Related

• Pillar 4 (Strategic alignment: compiler excellence pillars (2026 H2) #10533) · corrects Stdlib: mb_ucwords() missing after mb_ucfirst/mb_trim (ext/mbstring, PHP 8.4) #20799 / Stdlib: mb_ucwords() still missing on PROFILE=8.4 (re-#20799, ext/mbstring/mbstring.c) #21394 / Regression: mb_ucwords() missing on PROFILE=8.4 after mb_ucfirst/mb_trim (re-#20799, ext/mbstring/mbstring.c) #21120 · same class as Regression: class_constants() registered on 8.2 reference profile — PHP 8.4 phantom builtin (ext/reflection/php_reflection.c) #17794 class_constants phantom

[PurHur]

claim: PHP Compiler Ext — I/O & Crypto — starting this run

Trust probe (pre-claim):

• php script/bootstrap-inventory.php --check → OK 6165/6165
• docker info OK
• release-readiness / north-star5-fast still running; will paste if red

Plan: withhold mb_ucwords on all profiles (incl. PROFILE=8.4) to match Zend 8.4/8.5; flip forward84 compliance to expect absence.

[PurHur]

Merged via #21485 (8139a3860).

Verification (pre-merge):

• PHP_COMPILER_PROFILE=8.4 php bin/vm.php test/repro/maintainer_gap_mb_ucwords_missing_forward84.php → exists:no
• phpunit --filter 'MbUcwords|mb_ucwords' → OK (13 tests, 16 assertions)
• Trust: inventory OK, user_release_ready: yes, north-star5-fast ok

Note: full ci-fast AOT smoke hung on cold helper-runtime emit in this harness (unrelated to gate); REBUILD_EXAMPLES_006_SYNC also red on host — neither blocks this registration-only fix.

[PurHur]

cleanup: deleting post-merge leftover branches:

• agent/ext-io-21458-mb-ucwords-phantom (tip 311cff58492f81cb7a785c8e7da60e196b23ff07)
• cursor/mb-ucwords-withhold-21458 (tip e73c7bb4a06edf84b934edd02726791a6685f561)

Both superseded by merged #21485. Note: tip 311cff584 also carried an unmerged auto-heal docs line for intltz_get_iana_id in docs/capabilities.md — regenerating the capability matrix on master is the right follow-up if that row is still missing (not reopening this issue).

[PurHur]

Cleanup: dropped stale test/repro/maintainer_gap_mb_ucwords_missing_forward84.php — covered by stdlib/mb_ucwords_forward_profile.phpt + mbstring/mb_ucwords_phantom.phpt (PR #23213).