Final and Readonly Classes in PHP

and calling the $obj = new ChildClass('Antonio'); again would result in: Class App\Utils\ChildClass cannot extend final class App\Utils\ParentClass .

So the definition of final in php is:

The final keyword is used to prevent a class from being inherited and to prevent inherited method from being overridden.

1. So if we declare class method as a final then that method cannot be override by the child class.
2. Same as method if we declare class as a final then that class cannot be extended any more.

readonly class

readonly classes are added on PHP 8.2  .

final and readonly are two completely different concepts in PHP and have different implications when applied to a class.

As we saw earlier, making a class final means that it cannot be extended.

final class ParentClass{}

// Fatal error: Class ChildClass cannot extend final class ParentClass
class ChildClass extends ParentClass{}

In the other hand, making a class readonly means several things:

• All instance properties of a class are implicitly marked as readonly, and cannot be reassigned.
• Creation of dynamic properties is not allowed.
• Only a readonly class can extend another readonly class.
• readonly properties can be overridden

For example:

// PHP 8.2+
readonly class ParentClass
{
 public function __construct(
 public string $name
 ) {}
}

$obj = new ParentClass('Antonio');
echo 'Hello, My name is ' . $obj ->name; // "Hello, My name is Antonio"

If you try to re-assign a value to any object property after instantiation, it would throw the following error:

// PHP 8.2+
// Fatal error: Uncaught Error: Cannot modify readonly property ParentClass::$name
$obj ->name = 'Ndershkuesi';

This happens because all properties are implicitly readonly when you mark the class as readonly.

NOTE: readonly property cannot have default value.

final and readonly

You can also mark a class as both readonly and final as well, this means:

• All class properties are implicitly readonly .
• The class cannot be extended.

Example:

// PHP 8.2+
final readonly class ParentClass {
 public function __construct(
 public string $name
 ) {}
}

Usage:

$obj = new ParentClass('Antonio');
echo 'My name is ' . $obj->name; // "My name is Antonio"

And if you try to assign a value to any object property or if you try to extend the class then it would throw an error as follows:

// PHP 8.2+

// Fatal error: Uncaught Error: Cannot modify readonly property ParentClass::$name
$obj->name = 'Ndershkuesi';

// Fatal error: Class ChildClass cannot extend final class Shape
readonly class ChildClass extends ParentClass {}

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