Final classes and methods in PHP - Step-by-Step Execution

Concept Flow - Final classes and methods

Define final class

↓

Try to extend final class?

Yes→Error: Cannot extend final class

No↓

Define class with final method

↓

Try to override final method?

Yes→Error: Cannot override final method

No↓

Use class and methods normally

This flow shows how final classes cannot be extended and final methods cannot be overridden, causing errors if attempted.

Execution Sample

PHP

<?php
final class Base {
  public function greet() {
    return "Hello";
  }
}

class Child extends Base {}
?>

This code tries to extend a final class, which causes an error.

Execution Table

Step	Action	Code Line	Result	Error
1	Define final class Base	final class Base {...}	Class Base created	None
2	Define class Child extending Base	class Child extends Base {}	Attempt to create Child	Fatal error: Cannot extend final class Base
3	Execution stops	-	-	Error stops script

💡 Error occurs because Child tries to extend final class Base

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
Class Base	undefined	defined	defined	defined
Class Child	undefined	undefined	error (not defined)	error (not defined)

Key Moments - 2 Insights

Why does PHP give an error when trying to extend a final class?

Because final classes cannot be extended. As shown in execution_table step 2, PHP stops with a fatal error when Child tries to extend Base.

Can a final method be overridden in a child class?

No. If a method is declared final, any attempt to override it in a subclass causes an error, similar to final classes but at method level.

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 2?

A Child class is created successfully

B No error, program continues

C Fatal error because Child tries to extend final class Base

D Base class is deleted

💡 Hint

Check execution_table row with Step 2 for error details

According to variable_tracker, what is the state of Class Child after step 2?

A Defined and usable

B Undefined due to error

C Overridden Base class

D Final class

💡 Hint

Look at variable_tracker row for Class Child after Step 2

If the final keyword is removed from class Base, what changes in the execution table?

A Step 2 error disappears, Child class is created

B Step 1 causes error

C Step 3 error appears

D No change, error remains

💡 Hint

Final keyword prevents extension; removing it allows Child creation

Concept Snapshot

final class ClassName {
  // class body
}

final methods prevent overriding:
class A {
  final function f() {}
}

- final class: cannot be extended
- final method: cannot be overridden
- Trying to do so causes fatal error

Full Transcript

In PHP, a final class cannot be extended by any other class. If you try to create a subclass from a final class, PHP stops with a fatal error. Similarly, a method declared as final inside a class cannot be overridden by subclasses. This example shows a final class Base and an attempt to extend it with Child, which causes an error. Variables track that Base is defined but Child is not created due to the error. Remember, final keyword protects classes or methods from being changed by inheritance.