The “Final” Keyword

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 −

finalclassmyclass{/*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

Open Compiler

<?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 −

classmyclass{finalfunctionmyfunction(){/* 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 −

Open Compiler

<?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

Explore our latest online courses and learn new skills at your own pace. Enroll and become a certified expert to boost your career.

Constant with “final” Keyword

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

finalpublicconstNAME="My Class";

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

Open Compiler

<?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.

Open Compiler

<?php
   class ellipse {
  final public const PI=22/7;
  private float $a, $b;
  public function __construct($x, $y) {
     $this-&gt;a = $x;
     $this-&gt;b = $y;
  }
  final public function area() : float {
     return self::PI*$this-&gt;a*$this-&gt;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.