Language: readonly property hooks — set hook cannot assign to readonly backing store outside __construct (zend_compile.c) #4518

Category

language

Problem

PHP 8.4 property hooks on readonly properties forbid writes to the backing value outside __construct, even from the property's own set hook (only get may read). Zend rejects invalid hook bodies at compile time or runtime per zend_compile.c / hook dispatch.

This compiler preprocesses hooks to __phpc_property_set_* methods (lib/SourcePreprocessor/PropertyHooks.php) and dispatches via lib/VM.php / lib/JIT/PropertyHookDispatch.php (#3145). Gap: a set hook that assigns $this->prop = … on a readonly promoted property may succeed on VM/JIT when Zend fatals or throws.

php-src reference

• php/php-src Zend/zend_compile.c — readonly + hooks validation
• php/php-src Zend/zend_object_handlers.c — property write + hook interaction
• RFC: Property hooks

Repro

repro_readonly_hook_set.php:

<?php
class C {
    public readonly string $name {
        set (string $value) {
            $this->name = strtoupper($value);
        }
    }
    public function __construct(string $v) {
        $this->name = $v;
    }
}

$c = new C('hi');
try {
    $c->name = 'no';
} catch (Error $e) {
    echo get_class($e), ': ', $e->getMessage(), "\n";
}

Compare Zend vs this compiler:

php repro_readonly_hook_set.php
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php repro_readonly_hook_set.php'

Scope (this repo)

• lib/SourcePreprocessor/PropertyHooks.php — compile-time diagnostics where possible
• lib/VM.php — asymmetricPropertyWriteMessage / readonly guards in hook dispatch
• lib/JIT/PropertyHookDispatch.php — readonly write mask for hook bodies
• Compliance: test/compliance/cases/language/readonly_property_hook_set.phpt

Done when

• Post-construct assignment through set hook matches Zend (Error/exception class + message)
• Constructor initialization still works
• JIT/AOT same as VM (no MCJIT-only bypass)

Related

• Closed Language: PHP 8.4 property hooks — get/set on properties (Zend parity) #3145, Language: property hooks — JIT/AOT read/write dispatch (phase 2 of #3145) #4205 — property hooks baseline
• Language: readonly properties on non-readonly classes (PHP 8.1+) #3149, Language: readonly classes (PHP 8.2+) #1360 — readonly property / readonly class guards