What is the purpose of the keyword final in PHP?

The final keyword is used to prevent overriding classes, methods, and constants.

Usage:

• Classes: If a class is declared as final, it cannot be inherited.

  <?php
  final class Base {
      // ...
  }
  
  // Fatal error: Class Derived may not inherit from final class Base
  // class Derived extends Base {
  //     // ...
  // }
  

• Methods: If a class method is declared as final, it cannot be overridden in a subclass.

  <?php
  class Base {
      final public function process() {
          // ...
      }
  }
  
  class Derived extends Base {
      // Fatal error: Cannot override final method Base::process()
      // public function process() {
      //     // ...
      // }
  }
  

• Constants: Starting from PHP 8.1, class constants can also be declared as final. This prevents them from being overridden in subclasses.

  <?php
  class Base {
      final public const VERSION = '1.0';
  }
  
  class Derived extends Base {
      // Fatal error: Cannot override final constant Base::VERSION
      // public const VERSION = '1.1';
  }
  

Purpose of final:

• Security: Ensures that key logic or intended behavior of a class/method is not altered in subclasses.
• API Stability: Guarantees that a specific implementation of a method or class will not be modified, which is especially important when developing libraries.
• Performance (slightly): In some cases, it can allow the compiler optimizer to perform more aggressive inline optimizations, knowing that the method will not be overridden.