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To ensure the class property will never be modified/changed by itself or children or external calls we can now use a readonly prefix to class property declaration like so: public readonly string $propertyName = 'thisValueWillNeverChange;. The only place when we could override the readonly variable is inside the class constructor.
Convert any callable into Closure object to be returned and then used/called when needed. The next syntax (aka First class callable syntax) of $this->methodName(...) is identical in behavior as current: Closure::fromCallable([this, 'methodName']). Do not confuse this syntax to unpacking argument syntax of ...$args.
Currently we could only assign a default parameter value of null, array or primitive types in functions and methods. In PHP 8.1 we will be able to provide any custom class as default value for our expected function parameter. This includes defining a class variables inside the parameters as well.
final to prevent from overriding the constant by the child class. Of course current class constant functionality remains the same.