Language: undefined variable E_WARNING missing on read — casts/unary/arithmetic/?? RHS silent (Zend/zend_execute.c)

[PurHur]

Category

language · runtime · php-src-strict

Problem

Zend emits Warning: Undefined variable $name when a variable is read without being defined (and error_reporting includes E_WARNING). This compiler silently treats undefined locals as null in many fetch paths — no warning on stderr.

Verified 2026-06-20 (Docker php vs bin/vm.php):

Expression	Zend PHP 8.2	VM today
echo $x;	Warning, empty output	no warning, empty output
+$x / -$x	Warning, 0	0 no warning ❌
$x + 1	Warning, 1	1 no warning ❌
(int)$x, (string)$x, (array)$x	Warning + coerced value	coerced, no warning ❌
$x ?? $y (both undefined)	Warning on $y, result NULL	NULL, no warning ❌
isset($x) / empty($x)	no warning	no warning ✓ (correct)

Distinct from #10164 / #10096 (compact() key omission) and #273 (undefined array keys). This issue is local variable fetch semantics shared by VM/JIT/AOT.

php-src reference

• php/php-src Zend/zend_execute.c — ZEND_FETCH_R, ZEND_CHECK_UNDEFINED_VAR
• php/php-src Zend/zend_variables.c — undefined variable handling
• php/php-src Zend/zend_operators.c — ?? evaluates RHS only when LHS is null/unset

Repro

<?php
declare(strict_types=1);

var_export(+$x);
echo "\n";
var_export($x ?? $y);
echo "\n";
var_export((string)$x);
echo "\n";

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php -r "var_export(+\$x); echo PHP_EOL; var_export(\$x ?? \$y); echo PHP_EOL;" 2>&1
php bin/vm.php -r "var_export(+\$x); echo PHP_EOL; var_export(\$x ?? \$y); echo PHP_EOL;" 2>&1
'

Runtime	Expected
Zend	three Undefined variable warnings; values 0, NULL, ''
VM	same warnings + values (today: values only)

@ must suppress warnings (#9293 trigger_error path).

Scope (this repo)

Path	Work
lib/VM/ErrorReporter.php	undefinedVariable() — wire into fetch paths
lib/VM.php	operand fetch for unary/binary/cast/coalesce RHS
lib/JIT/VarFetchHelper.php, lib/JIT/CoalesceHelper.php	JIT must emit same warnings when not @-silenced
Tests	test/compliance/cases/runtime/undefined_variable_read_warning.phpt

PHP-in-PHP: warning text + guard in lib/VM/; JIT calls compiled PHP trampoline (see #10353 follow-up php-in-php issue).

Done when

• Repro emits Zend-matching Undefined variable warnings on VM (and JIT when in scope)
• isset/empty still silent; @ suppresses warnings
• ./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter undefined_variable_read_warning' green
• No new logic in runtime/*.c

Related

• Runtime: Undefined array/superglobal key — Zend notice parity #273 undefined array keys · php-in-php: JIT StringTriggerErrorJit — route trigger_error()/undefined-key warnings through VmErrorHandler PHP not LLVM (#1492) #9293 php-in-php error handler · php-in-php: CoalesceRuntime + NullsafeRuntime — route ??/?-> LLVM bridges through Vm PHP not lib/JIT/Builtin/*Runtime (#1492) #10311 coalesce JIT · Epic: Self-host critical path — compiler compiles itself (M3→M5) #1492

[PurHur]

claim: cursor-agent-lane-c — starting this run

[PurHur]

claim: cursor-agent-lane-c — starting this run (continuing agent/issue-10358-undefined-var-warning)

[PurHur]

Merged via #10388.

Fix: VM now emits Zend-matching Undefined variable E_WARNING when unbound locals/globals are read through casts, unary ops, binary arithmetic, echo, and ?? RHS paths. Extended unbound detection to {main} script globals via isGlobalEverAssigned.

Verification:

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter undefined_variable_read_warning'
# OK (2 tests, 2 assertions)

JIT parity follow-up remains #10360.