Serialize closures declared in constant expressions

[nicolas-grekas]

RFC: https://wiki.php.net/rfc/serializable_closures

Closures declared in constant expressions of a class member (anonymous closures and first-class callables, of any visibility) become serializable as references to their declaration site. unserialize() resolves the reference against the loaded class; payloads contain no code and can only designate closures that classes declare. Runtime-created closures keep refusing to serialize with the current error.

The public surface is serialize()/unserialize() alone, through the regular __serialize()/__unserialize() protocol; no new reflection API. Exporters recreate a closure from generated code by embedding the payload in an unserialize('O:7:"Closure":...') call, which goes through the same resolution and verification.

The __serialize() payload is [<object properties>, [<tag>, <reference>]] (the shape ext/uri and ext/random use), where the tag names the reference kind; only "const-expr" exists today. The reference is four fields, [class, site, key, hash]. site names the class member that declares the closure (a constant name, $prop, $prop::get()/$prop::set(), or method(), with parameters folded into their method; empty for an attribute on the class itself), so a reference stays valid when unrelated members of the class change. For an anonymous closure key is its integer rank among the element's closures; for a first-class callable it is the callable's name ("Called::method" / "function"), since the name is already the address.

hash guards an anonymous rank: a hash of the closure's pretty-printed code, computed at compile time and stored on the declaration's AST node (persisted by opcache), so a rank that comes to name a different closure (e.g. after two attributes on the same member are swapped) fails loudly instead of resolving silently. A producer that cannot compute it (a userland polyfill) passes 0 and the rank resolves positionally, unverified. First-class callables carry 0 too: a name cannot drift.