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.

Practice

1. What does the final keyword do when used with a class in PHP?

easy

A. Prevents the class from being extended by other classes

B. Allows the class to be extended multiple times

C. Makes the class abstract

D. Allows the class methods to be overridden

Solution

1. Step 1: Understand the role of final with classes

   The final keyword before a class means no other class can extend it.

2. Step 2: Compare with other options

   Options A, B, and D describe behaviors that are opposite or unrelated to final classes.

3. Final Answer:

   Prevents the class from being extended by other classes -> Option A

4. Quick Check:

   final class = no extension allowed [OK]

Hint: Final class means no child classes allowed [OK]

Common Mistakes:

• Thinking final class can be extended
• Confusing final with abstract
• Assuming final allows method overrides

2. Which of the following is the correct syntax to declare a final method in a PHP class?

easy

A. function final myMethod() {}

B. final function myMethod() {}

C. function myMethod() final {}

D. final myMethod function() {}

Solution

1. Step 1: Recall correct final method syntax

   In PHP, the final keyword comes before the function keyword when declaring a final method.

2. Step 2: Check each option

   final function myMethod() {} matches the correct syntax. Options A, C, and D have incorrect order or placement of keywords.

3. Final Answer:

   final function myMethod() {} -> Option B

4. Quick Check:

   final before function keyword = correct syntax [OK]

Hint: final keyword always before function name in declaration [OK]

Common Mistakes:

• Placing final after function name
• Writing final after function keyword incorrectly
• Mixing order of keywords

3. What will be the output of the following PHP code?

final class Car {
    final public function start() {
        return "Engine started";
    }
}

class SportsCar extends Car {
    public function start() {
        return "Sports engine started";
    }
}

$car = new SportsCar();
echo $car->start();

medium

A. Engine started

B. Sports engine started

C. Fatal error: Cannot override final method Car::start()

D. Fatal error: Cannot extend final class Car

Solution

1. Step 1: Analyze class and method declarations

   The class Car is declared as final, so it cannot be extended. But the code tries to extend it with SportsCar.

2. Step 2: Identify the error

   Since Car is final, extending it causes a fatal error. But the question is about overriding a final method. Actually, the class is final, so extending is not allowed at all.

3. Step 3: Check options carefully

   Fatal error: Cannot extend final class Car says cannot extend final class, which matches the error. Fatal error: Cannot override final method Car::start() says cannot override final method, which would be error if class was not final but method was final.

4. Final Answer:

   Fatal error: Cannot extend final class Car -> Option D

5. Quick Check:

   final class cannot be extended = fatal error [OK]

Hint: Final class cannot be extended, causes fatal error [OK]

Common Mistakes:

• Confusing final method error with final class error
• Assuming method override works despite final class
• Ignoring that class is final and cannot be extended

4. Identify the error in the following PHP code:

class Vehicle {
    final public function move() {
        return "Moving";
    }
}

class Bike extends Vehicle {
    public function move() {
        return "Bike moving";
    }
}

medium

A. No error, code runs fine

B. Cannot extend class Vehicle

C. Cannot override final method move() in class Bike

D. Syntax error in method declaration

Solution

1. Step 1: Understand final method behavior

   The method move() in Vehicle is declared as final, so child classes cannot override it.

2. Step 2: Check the child class method

   The Bike class tries to override move(), which is not allowed and causes a fatal error.

3. Final Answer:

   Cannot override final method move() in class Bike -> Option C

4. Quick Check:

   final method cannot be overridden [OK]

Hint: Final method cannot be changed in child classes [OK]

Common Mistakes:

• Thinking final method can be overridden
• Confusing final method with abstract method
• Assuming no error when overriding final method

5. You want to create a base class Document that cannot be extended, but you want to allow extending a method print() in child classes. Which is the correct approach?

hard

A. Do not declare class Document as final, and declare method print() normally

B. Declare class Document as final, and method print() normally (not final)

C. Do not declare class Document as final, but declare method print() as final

D. Declare class Document as final, and method print() as final

Solution

1. Step 1: Understand final class effect

   Declaring a class as final prevents any extension, so no child classes can exist to override methods.

2. Step 2: Understand method overriding

   To allow overriding print(), the class must be extendable and the method must not be final.

3. Step 3: Choose correct option

   Do not declare class Document as final, and declare method print() normally allows extending the class and overriding the method. Options B and D prevent extension. Do not declare class Document as final, but declare method print() as final prevents method overriding.

4. Final Answer:

   Do not declare class Document as final, and declare method print() normally -> Option A

5. Quick Check:

   Allow extension and method override = no final on class or method [OK]

Hint: Final class blocks extension; omit final to allow overrides [OK]

Common Mistakes:

• Declaring class final but expecting method override
• Declaring method final when override is needed
• Confusing final class and final method effects