Extending classes in PHP - Step-by-Step Execution

Concept Flow - Extending classes

Define Parent Class

↓

Define Child Class extends Parent

↓

Create Child Object

↓

Child inherits Parent methods/props

↓

Call methods on Child

↓

If method overridden, Child's version runs

↓

Program Ends

This flow shows how a child class inherits from a parent class, can override methods, and how objects use these methods.

Execution Sample

PHP

<?php
class Animal {
  public function speak() {
    echo "Animal sound\n";
  }
}

class Dog extends Animal {
  public function speak() {
    echo "Bark!\n";
  }
}

$dog = new Dog();
$dog->speak();
?>

This code defines a parent class Animal and a child class Dog that overrides the speak method. It creates a Dog object and calls speak.

Execution Table

Step	Action	Class/Method	Output	Notes
1	Define class Animal	Animal		Parent class created with speak() method
2	Define class Dog extends Animal	Dog		Child class created, inherits Animal
3	Create object $dog = new Dog()	Dog::__construct		Dog object created
4	Call $dog->speak()	Dog::speak	Bark!	Dog's speak() overrides Animal's
5	Program ends			Execution complete

💡 Program ends after calling Dog's speak method which outputs 'Bark!'

Variable Tracker

Variable	Start	After Creation	Final
$dog	undefined	object of Dog	object of Dog

Key Moments - 2 Insights

Why does $dog->speak() print 'Bark!' and not 'Animal sound'?

Because Dog class overrides the speak() method from Animal. See execution_table step 4 where Dog::speak runs instead of Animal::speak.

Does Dog class have access to Animal's methods if not overridden?

Yes, Dog inherits all methods from Animal. If Dog does not override a method, calling it on Dog object uses Animal's version.

Visual Quiz - 3 Questions

Look at the execution table, what is the output at step 4?

A Animal sound

B No output

C Bark!

D Error

💡 Hint

Check the Output column in execution_table row with Step 4

At which step is the Dog object created?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the Action column for object creation in execution_table

If Dog did not override speak(), what would $dog->speak() output?

A Bark!

B Animal sound

C No output

D Error

💡 Hint

Refer to key_moments about method inheritance and overriding

Concept Snapshot

Extending classes in PHP:
class Child extends Parent {
  // Child inherits Parent's methods and properties
  // Can override methods by redefining them
}
Create child object: $obj = new Child();
Calling methods uses Child's version if overridden, else Parent's.

Full Transcript

This example shows how PHP classes can extend other classes. The parent class Animal has a method speak() that prints 'Animal sound'. The child class Dog extends Animal and overrides speak() to print 'Bark!'. When we create a Dog object and call speak(), it runs Dog's version, printing 'Bark!'. This demonstrates inheritance and method overriding in PHP classes.

Practice

1. What keyword is used in PHP to create a child class that inherits from a parent class?

easy

A. extends

B. inherits

C. implements

D. uses

Solution

1. Step 1: Understand class inheritance in PHP

   PHP uses a specific keyword to let one class inherit properties and methods from another.

2. Step 2: Identify the correct keyword

   The keyword to extend a class in PHP is extends, not inherits, implements, or uses.

3. Final Answer:

   extends -> Option A

4. Quick Check:

   Keyword for inheritance = extends [OK]

Hint: Remember: PHP uses 'extends' to inherit classes [OK]

Common Mistakes:

• Using 'inherits' instead of 'extends'
• Confusing 'implements' with 'extends'
• Using 'uses' keyword incorrectly

2. Which of the following is the correct syntax to extend a class named Animal in PHP?

easy

A. class Dog extends Animal {}

B. class Dog inherits Animal {}

C. class Dog implements Animal {}

D. class Dog use Animal {}

Solution

1. Step 1: Recall PHP class extension syntax

   To create a child class, PHP uses the syntax: class Child extends Parent {}.

2. Step 2: Match the syntax with options

   class Dog extends Animal {} matches the correct syntax. The other options use incorrect keywords.

3. Final Answer:

   class Dog extends Animal {} -> Option A

4. Quick Check:

   Correct syntax uses 'extends' keyword [OK]

Hint: Use 'extends' keyword exactly after class name [OK]

Common Mistakes:

• Using 'inherits' instead of 'extends'
• Confusing 'implements' with 'extends'
• Using 'use' keyword for inheritance

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

class ParentClass {
    public function greet() {
        return "Hello from Parent";
    }
}

class ChildClass extends ParentClass {
    public function greet() {
        return "Hello from Child";
    }
}

$object = new ChildClass();
echo $object->greet();

medium

A. Hello from Parent

B. Fatal error

C. Hello from Child

D. No output

Solution

1. Step 1: Understand method overriding in child class

   The child class ChildClass overrides the greet() method of the parent class.

2. Step 2: Determine which method is called

   When calling greet() on an instance of ChildClass, the overridden method in the child class runs.

3. Final Answer:

   Hello from Child -> Option C

4. Quick Check:

   Child method overrides parent method [OK]

Hint: Child method overrides parent method with same name [OK]

Common Mistakes:

• Assuming parent method runs instead of child
• Expecting syntax error due to override
• Thinking no output will be printed

4. Identify the error in this PHP code that tries to extend a class:

class Vehicle {
    public function start() {
        return "Starting vehicle";
    }
}

class Car Vehicle {
    public function start() {
        return "Starting car";
    }
}

medium

A. Class Car should not have methods

B. Method start() cannot be overridden

C. Class Vehicle should be abstract

D. Missing 'extends' keyword before Vehicle

Solution

1. Step 1: Check class extension syntax

   In PHP, to extend a class, the child class must use the extends keyword.

2. Step 2: Identify the missing keyword

   The code class Car Vehicle {} misses extends between Car and Vehicle.

3. Final Answer:

   Missing 'extends' keyword before Vehicle -> Option D

4. Quick Check:

   Extending requires 'extends' keyword [OK]

Hint: Always write 'extends' when inheriting a class [OK]

Common Mistakes:

• Omitting 'extends' keyword
• Thinking methods cannot be overridden
• Confusing abstract class requirement

5. Given the following classes, what will be the output?

class Base {
    public function message() {
        return "Base message";
    }
}

class Derived extends Base {
    public function message() {
        return parent::message() . " and Derived message";
    }
}

class MoreDerived extends Derived {
    public function message() {
        return parent::message() . " plus MoreDerived message";
    }
}

$obj = new MoreDerived();
echo $obj->message();

hard

A. MoreDerived message

B. Base message and Derived message plus MoreDerived message

C. Base message

D. Fatal error due to parent call

Solution

1. Step 1: Trace method calls through inheritance chain

   MoreDerived::message() calls parent::message(), which is Derived::message().

2. Step 2: Understand Derived::message() call

   Derived::message() calls parent::message(), which is Base::message(), returning "Base message".

3. Step 3: Combine returned strings

   Base returns "Base message"; Derived appends " and Derived message"; MoreDerived appends " plus MoreDerived message".

4. Final Answer:

   Base message and Derived message plus MoreDerived message -> Option B

5. Quick Check:

   parent:: calls chain methods up inheritance [OK]

Hint: parent:: calls chain up inheritance to combine outputs [OK]

Common Mistakes:

• Ignoring parent:: calls and output only last method
• Expecting fatal error from parent calls
• Thinking only base method runs