Language: $a[missing]++ omits Undefined array key Warning — Zend warns then treats as null (zend_execute.c)

[PurHur]

Category

Language · php-src-strict · dim pre/post inc on undefined key

Problem

$a[0]++ when key 0 is undefined must emit Warning: Undefined array key 0, then treat the missing value as null and store 1. VM/JIT silently produce 1 with no Undefined array key Warning (final array matches).

Also visible after false→array auto-convert Deprecated: Zend still warns on the missing key; VM/JIT only show the Deprecated.

Probed 2026-08-11 — php:8.4-cli 8.4.24 vs PHP_COMPILER_PROFILE=8.4 @ a0e79a9651.

Repro	Zend 8.4.24	VM / JIT
$x=[]; $x[0]++;	Warning: Undefined array key 0 then done / [0=>1]	no Warning; done / [0=>1]
$x=false; $x[0]++;	Deprecated: Automatic conversion of false to array… and Warning: Undefined array key 0 then [0=>1]	Deprecated only; [0=>1]

php-src reference

• php/php-src Zend/zend_execute.c / zend_vm_def.h — ZEND_PRE_INC / ZEND_POST_INC dim fetch undefined key Warning
• Related dim-read wording (Language: PROFILE≥8.3 dim-read Warning still "value of type …" — Zend "on false|true|int|…" (re-#30028, zend_execute.c) #30053) is PROFILE≥8.3 message shape — this issue is missing the undefined-key Warning on inc/dec

PHP implementation target

• Dim inc/dec fetch: emit Undefined array key Warning (E_WARNING) before coercing missing to null
• lib/VM/ + JIT dim-inc/dec paths
• PHP-in-PHP; no new runtime/*.c

Repro

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
export PHP_COMPILER_PROFILE=8.4
printf "%s\n" "<?php
\$x=[];
try { \$x[0]++; echo \"done\n\"; var_export(\$x); echo PHP_EOL; } catch (Throwable \$e) {
  echo get_class(\$e), \": \", \$e->getMessage(), PHP_EOL;
}
" >/tmp/dim_inc_undef.php
php -d error_reporting=E_ALL -d display_errors=1 bin/vm.php /tmp/dim_inc_undef.php
php -d error_reporting=E_ALL -d display_errors=1 bin/jit.php /tmp/dim_inc_undef.php
'

Done when

• VM + JIT emit Undefined array key Warning for $a[missing]++ / -- (empty array case)
• After false→array Deprecated, the same Warning still appears
• Final value remains 1 (php-src-strict)
• Compliance .phpt under test/compliance/cases/
• php-src-strict; no php-compiler-strict shortcut

[PurHur]

claim: worker-lane-A — starting this run

Pillar probes (pre-claim):

• docker info: ok
• bootstrap-inventory --check: ran (file-list/construct-flag drift vs committed doc; exit 0 path still printed regenerate hint)
• release-readiness --json: in flight

Focus: emit Undefined array key Warning on dim pre/post-inc when key missing (php-src-strict).

[PurHur]

claim: worker-lane-A — implementing dim inc/dec undefined array key Warning (#30078)

[PurHur]

Merged via #30158.

Verification: issue repro green on VM + JIT (PHP_COMPILER_PROFILE=8.4) — \$x[0]++ and \$false[0]++ now emit Undefined array key 0 (plus false→array Deprecated on the second case); final arrays remain [0=>1].

Compliance: test/compliance/cases/language/dim_inc_undefined_array_key_warning.phpt.

Not run: full VMTest/JITTest shards, bootstrap/self-host gates (language-only change).