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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.
ReflectionMethod or class property ReflectionProperty we can now set its accessibility and call it manually without getting any errors.
final to prevent from overriding the constant by the child class. Of course current class constant functionality remains the same.
Every PHP internal methods from internal Classes will now contain a return type declarations. This change will impact any case of extending and overriding of internal PHP classes with custom implementations. From 8.1 we will get a deprecation and since 9.0 it will be a Fatal Error.
Local method static variables will now be inherited between multiple objects and even classes if base Class is used as parent Class. Previously each local static var was only related to the current type of Class.
Since PHP 7.4 magic methods of __seralize() and __unserialize() were added, since then Serializable interfece is obsolete and in fact broken with new PHP 8.x features. This also includes deprecating of PDO::FETCH_SERIALIZE flag of PDO object that will be remove completely in PHP 9.
Read morePHP 8.1 – Asynchronous and lightweight thread execution: Fibers
For any methods or function which are meant to always throw exception or exit the code using things like: exit() or die() a new return type of: noreturn was added. The sample usage could be: function iWillAlwaysExit(): noreturn .