PHP - The "Final" Keyword

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The "final" keyword in PHP is used in the definition of a class, a method inside a class, as well as with the definition of a constant property of a class.

A Class with "final" Keyword

Let's see how to create a class with the "final" keyword −

final class myclass {
   /*class members*/   
}

The "final" keyword in class definition prevents such a class from being extended. In other words, you cannot use a final class as a parent. If you try, PHP parser throws an error

<?php
   final class myclass {
   
      /* class body */
   }
   class newclass extends myclass {
   
      /* class body */ 
   }
?>

When you run this code, it will show an error −

PHP Fatal error:  Class newclass may not inherit from final class (myclass)

Method with "final" Keyword

Here is how you can create a method with the "final" keyword −

class myclass {
   final function myfunction() {
   
      /* function body */
   }
}

Prefixing a method definition with the final keyword prevents it from being overridden in a child class. A class with final method can be extended, but the child class cannot override it.

Example

Take a look at the following example −

<?php
   class myclass {
      final public function hello() {
         echo "Hello World!";
      }
   }
   class newclass extends myclass {
      public function hello() {
         echo "Hello PHP!";
      }
   }
?>

When you run this code, it will show an error −

PHP Fatal error:  Cannot override final method myclass::hello() in hello.php

Constant with "final" Keyword

You can also declare a constant in a class with the final keyword, starting from PHP 8.1.0 onwards.

final public const NAME = "My Class";

If you try to override a final constant from parent class in a child class, an error is encountered.

<?php
   class myclass {
      final public const NAME = "My Class";
      final public function hello() {
         echo "Hello World!";
      }
   }

   class newclass extends myclass {
      public const NAME = "New Class";
   }
?>

When you run this code, it will show an error −

Fatal error: newclass::NAME cannot override final constant myclass::NAME

Example

The following PHP script contains a parent class ellipse with a PI constant and area() method both declared as final. They are inherited by the circle class. The area() function calculates the area of circle.

<?php
   class ellipse {
      final public const PI=22/7;
      private float $a, $b;
      public function __construct($x, $y) {
         $this->a = $x;
         $this->b = $y;
      }
      final public function area() : float {
         return self::PI*$this->a*$this->b;
      }
   }
   class circle extends ellipse {
      public function __construct(float $x) {
         parent::__construct($x, $x);
      }
   }
   $c1 = new circle(5);
   echo "Area: " . $c1->area() . PHP_EOL;
?>

It will produce the following output −

Area: 78.571428571429

Note that the instance variables or properties of a class cannot be declared as final.