feat(php-rpc): query real PHP version and binary for --version

Add a minimal shirabe-php-rpc crate that spawns the system PHP as a
child process and asks it for runtime information over a Unix domain
socket, then use it to fill the `--version` PHP line with the real
\PHP_VERSION and \PHP_BINARY instead of fixed placeholder values.

See docs/dev/php-rpc.md for the design and scope.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

1 files changed, 60 insertions, 0 deletions

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+# PHP RPC
+
+Composer can require a specific PHP version or loaded extensions, i.e., platform requirements.
+To mimic this behavior needs a real PHP runtime.
+
+This document describes a first design of PHP runtime: a `shirabe-php-rpc` crate that spawns the
+system PHP as a child process and asks it for runtime information over a Unix domain socket.
+
+## Scope
+
+The crate supports exactly one interaction pattern, and nothing else:
+
+> Rust calls a named, argument-less PHP function and receives a single fixed-type scalar back.
+
+- Rust to PHP only. PHP never calls back into Rust.
+- No arguments.
+- Scalar return values only (string / int / float / bool / null).
+- Every failure is ignored: a PHP exception, serialization/deserialization
+  failure, a missing PHP function, a crashed child, etc. None are handled.
+  If anything goes wrong, behavior is undefined.
+- Windows is unsupported and `panic!`s for now.
+
+## Locating PHP
+
+Reuse the existing `PhpExecutableFinder` class to resolve the PHP binary.
+
+## Transport
+
+- A Unix domain socket. (No Windows support for now)
+- The PHP glue code is a small script written to a temporary file.
+- Message frame: `[usize length (little-endian)][payload]`.
+  - Request payload: the bare PHP function name as raw bytes.
+  - Response payload: `serialize()` of the function's return value.
+
+The PHP worker is a single read-eval-respond loop: read a framed function name,
+call the matching entry in a fixed dispatch table, send back `serialize($result)`.
+
+## Global state and public API
+
+PHP runtime information (e.g., process handle) is held as process-global state
+rather than threaded through call sites for now.
+The crate exposes plain free functions:
+
+```rust
+shirabe_php_rpc::get_php_version() -> String
+```
+
+The connection is a process-global `static` (e.g. `OnceLock<Mutex<Worker>>`), lazily initialized on
+the first call: the first `get_php_version()` spawns the child, performs the handshake, and caches
+the connection. Commands that never query PHP never start it. The child lives for the rest of the
+process and is left to be reaped at exit (no explicit shutdown message).
+
+A future revision threads this runtime information through arguments or embeds it in structs; for now
+callers just reach for the global getter.
+
+## Out of scope
+
+Deferred things: arguments and non-scalar return values, PHP to Rust callbacks
+and re-entrancy, object handles / proxies / identity, stub generation, error
+propagation, GC / lifecycle, and Windows support.