fix: move const inside class to prevent memory leaks in worker mode

[zip-fa]

💡 Motivation and Context

PHP applications running in long-lived worker modes (FrankenPHP, Swoole, RoadRunner, Laravel Octane) suffer from memory leaks caused by namespace-level constants in lib/Client.php and lib/FeatureFlag.php.

When SIZE_LIMIT and LONG_SCALE are declared at the namespace level with const, PHP re-evaluates and allocates them on every request in persistent worker processes, since namespace constants are not garbage-collected between requests in these runtimes. Moving them to private const on their respective classes ties their lifecycle to the class autoloader cache, which is loaded once and reused — eliminating the leak.

💚 How did you test it?

• Ran a Laravel Octane (FrankenPHP worker mode) application with PostHog SDK under load and monitored memory_get_usage() over 10,000+ requests — memory remained stable after the fix, whereas it grew linearly before.
• Verified all existing unit tests pass (composer test).
• Confirmed self::SIZE_LIMIT and self::LONG_SCALE resolve to the same values as the original namespace constants.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

Fixes #153

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix: move const inside class to prevent ..." | Re-trigger Greptile}

[marandaneto]

@zip-fa can you add a changeset pls? https://github.com/PostHog/posthog-php/blob/main/RELEASING.md
thansk for the pr

[zip-fa]

@marandaneto done