PHP Final Keyword

The final keyword in PHP is used to prevent class inheritance or method overriding. There are two main uses of the final keyword:

1. Preventing Class Inheritance: When you declare a class as final, it cannot be extended (inherited) by any other class.
2. Preventing Method Overriding: When you declare a method as final, it cannot be overridden in any subclass.

Here are examples to demonstrate both uses:

Example 1: Preventing Class Inheritance

<?php
final class BaseClass {
    public function sayHello() {
        echo "Hello from BaseClass!";
    }
}

// This will cause an error
class ChildClass extends BaseClass {
    public function sayHello() {
        echo "Hello from ChildClass!";
    }
}

$base = new BaseClass();
$base->sayHello();

// Uncommenting the following lines will cause an error
// $child = new ChildClass();
// $child->sayHello();
?>

Output:

Hello from BaseClass!

If you try to uncomment the ChildClass definition, PHP will throw an error:

Fatal error: Class ChildClass may not inherit from final class (BaseClass)

Example 2: Preventing Method Overriding

<?php
class BaseClass {
    public final function sayHello() {
        echo "Hello from BaseClass!";
    }
}

class ChildClass extends BaseClass {
    // This will cause an error
    public function sayHello() {
        echo "Hello from ChildClass!";
    }
}

$base = new BaseClass();
$base->sayHello();

// Uncommenting the following lines will cause an error
// $child = new ChildClass();
// $child->sayHello();
?>

Output:

Hello from BaseClass!

If you try to uncomment the sayHello method in ChildClass, PHP will throw an error:

Fatal error: Cannot override final method BaseClass::sayHello()

Summary

• Declaring a class as final prevents other classes from inheriting it.
• Declaring a method as final prevents subclasses from overriding it.

Using the final keyword is useful when you want to ensure certain parts of your code remain unchanged or unextended to maintain stability and consistency in your application.