Deprecated: Constant MHASH_XXH32 is deprecated since PHP8.5

[mbilykov]

Environment

• Rector: 2.5.8 (latest as of 2026-07-27)
• PHP: 8.5.8 (CLI)
• OS: Linux (Docker, php:8.5-cli based image)

Bug description

Since PHP 8.5, the legacy MHASH_* constants (mhash extension) are deprecated
(php/php-src#16569 — the constants were overlooked when mhash_*() functions
were deprecated in PHP 8.1).

DowngradeHashAlgorithmXxHashRector references these constants in a class
constant array, which triggers a E_DEPRECATED notice the moment the class
is loaded / its constants are evaluated — even when the rule never matches
any code in the analyzed project. On PHP 8.5 this producing a wall of noise on
every rector process invocation.

Steps to reproduce

1. Use PHP 8.5 as CLI.
2. Run vendor/bin/rector process <path> with any config that includes the
   PHP 8.1 downgrade set (withDowngradeSets(php81: true) or any lower
   target, e.g. php74).
3. Observe repeated deprecation notices in stdout.

Actual output

Deprecated: Constant MHASH_XXH32 is deprecated since 8.5, as the mhash*() functions were deprecated in vendor/rector/rector/vendor/rector/rector-downgrade-php/rules/DowngradePhp81/Rector/FuncCall/DowngradeHashAlgorithmXxHashRector.php on line 35
(repeated for MHASH_XXH64, MHASH_XXH3, MHASH_XXH128, each x2)

Expected behavior

No deprecation notices from Rector's own internals on PHP 8.5.

Root cause

rules/DowngradePhp81/Rector/FuncCall/DowngradeHashAlgorithmXxHashRector.php, line 35:

private const HASH_ALGORITHMS_TO_DOWNGRADE = [
    'xxh32'  => \MHASH_XXH32,
    'xxh64'  => \MHASH_XXH64,
    'xxh3'   => \MHASH_XXH3,
    'xxh128' => \MHASH_XXH128,
];
These MHASH_* int values are only used internally as unique sentinel
values for array_search() (line ~143, mapConstantToString()) — the rule
maps hash()'s string algo name to/from a constant-fetch AST node. The
actual mhash extension values are irrelevant here; only uniqueness
matters. There is no dependency on the real mhash constant value anywhere
in this rule.