Add support for PHP resp-bench

[prateek-kumar-improving]

Implements the PHP benchmark engine, at parity with the Java (reference), Ruby, and C# engines.

What

• New engine under php/ (composer install, php bin/resp-bench), driving:
  • valkey-glide-php — the Valkey GLIDE PHP client (native ext-valkey_glide extension, PHPRedis-compatible API)
  • phpredis — PHPRedis (ext-redis), the de-facto standard PHP client and the incumbent comparison baseline (analogous to redis-rb for Ruby / StackExchange.Redis for C#)
  • recording — in-memory driver for server-free tests
• Consumes the shared driver/workload JSON and emits the exact NDJSON schema (metadata/phase/totals/metrics, unit:"us", uppercased command keys, HDR compressed base64).

Concurrency model

PHP's GLIDE client is synchronous and the standard PHP build is non-thread-safe (NTS), which rules out ext-parallel. The engine therefore uses process-per-connection via pcntl — the faithful, dependency-free analogue of Java's virtual-thread-per-client and Ruby's thread-per-client:

• connections = N → fork N worker processes (capped at 256), each with one in-flight request at a time (the client == connection invariant), keeping results comparable across engines.
• Each worker connects after forking — a connection is never inherited across a fork (required for correctness with the native extension).
• Workers stream partial metrics to the parent over a stream_socket_pair; the parent reconstructs and merges the HdrHistograms losslessly (sparse bucket counts) before writing NDJSON.
• Phase-level rps_limit is divided across workers so the aggregate matches the target.
• An inline mode (--concurrency inline) runs all connections sequentially in one process; it's selected automatically for the recording driver and when pcntl is unavailable, and exercises the full pipeline for server-free tests.

Cross-engine parity

• JavaRandom LCG port — verified byte-identical to Java's canonical new Random(0).nextInt() sequence [-1155484576, -723955400, 1033096058, -1690734402, -1557280266, ...]. PHP lacks 64-bit integer overflow wraparound, so the multiply uses a 24-bit split to preserve the low 48 bits.
• sequential_int walks the keyspace; uniform_rand seeds per worker (seed + idx) for reproducible-yet-distinct sequences. Key formatting %0Nd honoring key_size_bytes.
• Pure-PHP HdrHistogram (1, 600_000_000, 3); the V2 compressed base64 payload is byte-structured to match Java's encodeIntoCompressedByteBuffer(). Leaky-bucket rate limiter.

Harness

• Makefile php-build/test/integration-test/run/clean/info targets.
• Registered the PHP driver_id in scripts/run_benchmark_matrix.py (DRIVER_ENGINE_MAP) and scripts/generate_graphs.py (DRIVER_LANGUAGE_MAP).
• Driver configs for both clients under configs/drivers/{default,high-throughput}/ + example-*-standalone.json, plus configs/matrices/php-driver-comparison.json (GLIDE-PHP vs PHPRedis across connection counts — the head-to-head that mirrors the Java/Node comparison matrices).
• CI: a fast server-free test-php job (unit + integration) and a benchmark-php job that builds the valkey_glide extension from source and runs against a live server; wired into generate-graphs needs.
• Docs: php/README.md, docs/BENCHMARKS_PHP.md, README "Supported Languages" + Make Targets updates.

Tests

58 tests (cd php && vendor/bin/phpunit): parity anchors (JavaRandom seed-0 Java anchor, key generator), rate limiter, command selector, config loader, client factory, HDR encode structure + percentiles; integration tests covering the recording-driver pipeline (NDJSON schema, inline + process fork modes, agreement on totals), rate limiting (RPS enforcement, shared limit across concurrent connections, unlimited throughput), error metrics (per-command error counts, errors excluded from latency histogram), and NDJSON/metrics output. Gated live tests for both valkey-glide-php and phpredis skip cleanly without their extension + a server. No server needed for the 50 non-live tests.

Verified locally (PHP 8.5, no server, no extension): full suite = 58 tests pass, 8 skipped (the gated live tests for the two real drivers). CLI smoke test with the recording driver produces valid NDJSON with correct totals (WARMUP 400, STEADY 1000 = GET+SET) and non-empty HDR payloads in both inline and --concurrency process (fork) modes.

Notes

• valkey-glide-php follows the valkey-glide-ruby / valkey-glide-csharp naming convention (not the bare valkey-glide, which is Java's).
• Live-server runs require the valkey_glide extension installed (php/README.md documents install via pie/PECL/source; the benchmark-php CI job builds it). The recording-driver e2e covers the engine paths without it.
• The HDR payload's binary structure was verified (cookies, header fields, IEEE754 conversion ratio), but a live cross-decode against Java's decodeFromCompressedByteBuffer() was not run in this environment (no JRE); it's ported from the Java-validated Ruby encoder.