RFC: Locked Classes

Hi all, I'd like to present a new RFC for "locked classes": classes which restrict dynamically adding or removing properties from their instances. While it can be useful, the ability to set an object property which is not part of the class definition can also lead to subtle bugs. Banning this for all objects would be a significant and painful breaking change, so I propose instead the option to mark a particular class with a new keyword, "locked". An instance of a locked class behaves like any other object, except that: - Attempting to set a property on the instance which was not declared in the class (or inherited from one of its parent classes) will throw an error, and the instance will not be modified. - Attempting to read a property on the instance which was not declared (or inherited) will throw an error, rather than raising a Notice and evaluating to null. - Attempting to call unset() on any property of the instance will throw an error, and the instance will not be modified. Note that ECMAScript  / JavaScript includes a similar feature, called "sealed objects". However, the proposed modifier applies to classes, and "sealed class" has a different meaning elsewhere (e.g. C#, Kotlin), so I've chosen "locked class" to avoid confusion. For further details and examples, please check the RFC at https://wiki.php.net/rfc/locked-classes and the tests in the draft implementation at https://github.com/php/php-src/pull/3931 Regards, -- Rowan Collins [IMSoP]