[FFI] Opt-in mapping of C struct types to userland PHP classes (typed CData handles)

Description

Feature request

PHP FFI represents every C value — a struct zend_string*, a zval*, a
char*, an int — as one and the same final class, FFI\CData. That single
opaque type is what makes FFI so flexible, but it also means no C struct a
binding works with can ever be described to static analysis or an IDE. There
is no way to say "this handle is a zend_string, these are its fields", and no
way to make $handle instanceof ZendString true. FFI\CData being final
closes off every userland workaround.

This proposes an opt-in, per-scope class map: when you create an FFI scope
you may declare that C type X should be represented as instances of your class
\My\X (class extending FFI\CData), so that FFI::new('X'),
FFI::cast('X', …), struct-field reads and function returns all produce
\My\X instances. Nothing changes for anyone who does not ask for it.

Motivation — a concrete, load-bearing case study

z-engine drives the Zend Engine's own
internals through FFI. It dereferences dozens of engine structs —
zend_string, zend_function, zend_class_entry, zval, zend_op_array, …
— and every one of them is FFI\CData. To recover any static typing and IDE
autocompletion the project currently has to ship all of the following:

1. a code generator that slices each struct out of the PHP headers via clang and
   emits one analysis-only PHP stub class per struct (with @property/typed
   properties mirroring the C fields), https://github.com/lisachenko/z-engine/blob/8.4/stubs/zend-engine-structs.php
2. a .phpstorm.meta.php map so PhpStorm resolves the FFI entry points, https://github.com/lisachenko/z-engine/blob/8.4/.phpstorm.meta.php
3. a PHPStan dynamic-return extension so the analyser resolves them too, https://github.com/lisachenko/z-engine/blob/8.4/tools/phpstan/TypedEntryPointReturnExtension.php
4. a hand-maintained convention that every one of those stub classes is
   never loaded at runtime (they exist only for the analyser), because they
   cannot actually back the CData handles.

That is four moving parts, per project, to emulate one feature the runtime
could provide directly - and it is strictly weaker than the real thing: the
stub classes can never make instanceof work, can never enforce a parameter
type, and drift from the real ABI unless regenerated. Every FFI binding
generator (SWIG-style wrappers, FFIMe, hand-written bindings over libgit2,
libsodium, SDL, …) hits the same wall. A native class map solves it once, for
everyone, in ~the same amount of C code these projects spend working around it.

Proposal

An optional class map attached to an FFI scope, mapping C struct/union type
names to userland classes:

The scope takes an optional array $options configuration, in the spirit of
SoapServer/SoapClient (which accept a classmap, and SoapClient also a
typemap). Two keys are recognised — classmap (C type → userland class) and
typemap (C type → conversion callbacks):

$ffi = FFI::cdef($cCode, $lib, options: [
    'classmap' => [
        'zend_string' => \My\Engine\ZendString::class,
        'zend_value'  => \My\Engine\ZendValue::class,
    ],
    'typemap' => [
        // C type name => how to marshal it to/from PHP (for types that should
        // surface as something other than a raw CData handle)
        'zend_bool' => [
            'from_cdata' => fn(FFI\CData $c): bool => $c->cdata !== 0,
            'to_cdata'   => fn(bool $v, FFI\CData $c): void => $c->cdata = $v ? 1 : 0,
        ],
    ],
]);

final class ZendString extends \FFI\CData
{
    // Fields may be exposed as typed property hooks over the raw CData, and the
    // class may carry ordinary methods.
    public int $len { get => $this->readUint32('len'); }

    public function toPhpString(): string { /* ... */ }
}

Rules for a classmap class:

• it must extend FFI\CData,
• it may declare typed property hooks whose bodies read/write the underlying
  C fields through the raw CData, and it may declare methods ; the object's storage stays ext/ffi's
  zend_ffi_cdata, so a hook body operates on the raw structure rather than on a
  real backing store.

Given the map, every handle ext/ffi mints for a mapped C type — from
FFI::new(), FFI::cast(), FFI::addr(), a struct-field read that yields a
nested struct/pointer, or a function return value — is created as an instance of
the mapped class instead of the bare FFI\CData. get_class() is truthful,
instanceof works, and native parameter/return type declarations
(function f(ZendString $s)) are enforced by the engine. Field access, casting,
FFI::sizeof(), garbage collection and every other behaviour are byte-for-byte
identical to today — the object still is a zend_ffi_cdata, only its ce
differs.

Implementation sketch

The change is localized to ext/ffi and is zero-overhead when unused:

• Registry. Each zend_ffi scope gains a HashTable *class_map keyed on the
  resolved zend_ffi_type * (populated from options['classmap'] at cdef/load
  time by resolving each declared type name to its zend_ffi_type, and validating
  the target class extends zend_ffi_cdata_ce), plus an optional parallel
  typemap table of conversion callbacks. Both are NULL/empty for every
  existing user.
• Minting. Today every cdata is created with
  object_init_ex(&zv, zend_ffi_cdata_ce) (in zend_ffi_cdata_to_zval() and
  the FFI::new/FFI::cast method handlers). Wrap that single choice: when the
  active scope's class_map is non-empty, look up the value's
  zend_ffi_type *, and if a class is registered use it instead of
  zend_ffi_cdata_ce. One hash lookup, guarded by class_map != NULL, so the
  common path is unchanged.
• Layout & lifetime. The allocated object stays zend_ffi_cdata; only the
  std.ce pointer changes. All zend_ffi_cdata_handlers are shared, so GC,
  free, clone, and the read/write paths need no changes — this is what keeps the
  patch small and safe.
• Preloading. For opcache.preloaded scopes the map must be re-resolved per
  request (the zend_ffi_type * pointers are request/persistent-scoped); the
  natural place is alongside the existing per-request scope materialization.
• Struct classes carrying methods / property hooks. Because the mapped class
  is an ordinary ce (only the object storage is zend_ffi_cdata), methods and
  typed property hooks work with no extra machinery — a hook body just reads or
  writes the underlying C field through the raw CData.
• Unchanged: serialization stays forbidden (as for any cdata).

Backward compatibility

Fully opt-in and additive. No existing FFI program changes behaviour; the new
options array (with its classmap/typemap keys) is the only surface, and it
defaults to "no mapping". The only relaxation is that FFI\CData becomes
extendable for registered classes only — a normal class X extends FFI\CData
without registration can stay an error (or be allowed as an inert never-minted
class, whichever the RFC prefers).

Target & offer

I'd like to target PHP 8.6, ahead of feature freeze, and I'm volunteering to
write the implementation PR. I'd welcome feedback on the proposal

ext/ffi で zend_ffi_cdata_to_zval() と FFI::new()/FFI::cast() のハンドラを読むところから始め、次に cdef/load を通じたスコープの作成と、リクエストごとの preload の実体化を追跡します。実装には、合意された classmap/typemap の設計、マッピングされた CData インスタンス、およびオプションのないスコープでの変更されない動作が必要になります。