PHP - The Final Keyword

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In PHP, the final keyword prevents classes and functions from being changed or overridden. It helps to keep important parts of your code secure, to guarantee no one accidentally breaks them while making changes.

The "final" keyword 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.

What Does the final Keyword Do?

The final keyword can be used in two ways −

• For Classes: This prevents the class from being extended (inherited).

• For methods: This prevents them from being overridden in child classes.

In this chapter we will cover the below topics −

• Using final with Classes

• Using final with Methods

• Using final with Constants

• Real-World Example: ellipse and circle Classes

Using final with Classes

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

final class myclass {
   /*class members*/   
}

Example: Class inheritance error

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 */ 
   }
?>

Output

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

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

Using final with Methods

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: Method overriding error

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!";
      }
   }
?>

Output

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

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

Using final with Constants

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";

Example: Final constant inheritance error

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";
   }
?>

Output

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

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

Real-World Example: ellipse and circle Classes

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;
?>

Output

It will produce the following output −

Area: 78.571428571429

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