Extending classes in PHP - Mini Project: Build & Apply

Extending Classes in PHP

📖 Scenario: You are creating a simple program to manage vehicles. You want to reuse common features of vehicles but also add special features for cars.

🎯 Goal: Build a PHP program that defines a base class Vehicle and a child class Car that extends Vehicle. You will add properties and methods step-by-step and then display the car details.

📋 What You'll Learn

Create a base class called Vehicle with a property $brand.

Create a child class called Car that extends Vehicle.

Add a property $model to the Car class.

Add a method getDetails() in Car that returns a string with brand and model.

Create an object of Car and print the details.

💡 Why This Matters

🌍 Real World

Extending classes helps reuse code and organize related features in programs like vehicle management systems.

💼 Career

Understanding class inheritance is essential for PHP developers building scalable and maintainable applications.

1

Create the base class Vehicle

Write a PHP class called Vehicle with a public property $brand set to the string "Toyota".

PHP

<?php

// Create class Vehicle with public property $brand set to "Toyota"
// Your code here

Hint

Use class Vehicle { public string $brand = "Toyota"; } to create the class and property.

2

Create the Car class extending Vehicle

Add a class called Car that extends Vehicle. Inside Car, add a public property $model set to "Corolla".

PHP

<?php
class Vehicle {
    public string $brand = "Toyota";
}

// Create class Car that extends Vehicle
// Add public property $model set to "Corolla"
// Your code here

Hint

Use class Car extends Vehicle { public string $model = "Corolla"; }.

3

Add getDetails() method to Car

Inside the Car class, add a public method called getDetails() that returns a string combining $this->brand and $this->model separated by a space.

PHP

<?php
class Vehicle {
    public string $brand = "Toyota";
}

class Car extends Vehicle {
    public string $model = "Corolla";

    // Add public function getDetails() that returns brand and model
    // Your code here
}

Hint

Define public function getDetails(): string { return $this->brand . " " . $this->model; }.

4

Create Car object and print details

Create an object called $myCar of class Car. Then print the result of calling $myCar->getDetails().

PHP

brand . " " . $this->model;
    }
}

// Create object $myCar of class Car
// Print $myCar->getDetails()
// Your code here

Hint

Use $myCar = new Car(); and print($myCar->getDetails());.

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