Add PHP engine

[jamesx-improving]

Note

Copied from ikolomi/resp-bench#15.
Originally opened by @jeremyprime on 2026-04-09T17:38:39Z.

Summary

Add a PHP benchmark engine with support for the following drivers:

• valkey-glide
• phpredis (C extension)

This enables benchmarking GLIDE PHP against the most popular PHP Redis client.

Implementation Notes

• Follow the ADDING_LANGUAGE.md guide
• PHP is typically process-based; concurrency model will likely need to use multiple processes (similar to Ruby engine's forked approach)