Language: final class constants (final const) — parse + override rules (zend_compile.c parity) #4455

Category

language

Problem

PHP 8.1+ allows final const on classes/interfaces/traits/enums. Child classes cannot redefine or override a final const; Zend emits a compile-time error.

This compiler accepts ordinary const on classes (#2199) but does not model final on class constants — php-parser/php-cfg may parse the modifier without enforcing Zend inheritance rules.

php-src reference

• Zend/zend_compile.c — zend_compile_const_decl, ZEND_ACC_CONSTANT + final bit
• Zend/zend_inheritance.c — constant override checks
• RFC: Final class constants (PHP 8.1)

Repro (today)

<?php
class Base {
    final const X = 1;
}
class Child extends Base {
    const X = 2;
}

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

Zend PHP: compile-time Error (cannot override final constant Base::X)
This compiler: may compile and run, printing 2 — incorrect.

Also verify final const on interfaces/traits and fetch semantics Child::X.

Scope (this repo)

Done when

• Child redefinition is compile-time fatal with Zend-compatible message.
• Non-final constants still overridable per Zend rules.
• ./script/ci-fast.sh --filter final_class_const green.