Constructor & Inheritance in PHP

In the previous lesson, you created classes and objects and manually assigned values to properties.

But that approach has a problem:

$product = new Product();
$product->name = "Laptop";
// Forgot price

👉 This creates incomplete objects and leads to bugs.

Constructors and inheritance solve this by making object creation structured and reliable.

What is a Constructor?

A constructor is a special method that runs automatically when an object is created.

Creating a Constructor

class Product {
    public $name;
    public $price;

    public function __construct($name, $price) {
        $this->name = $name;
        $this->price = $price;
    }
}

Creating Object with Constructor

$product = new Product("Laptop", 50000);

echo $product->name;

Output:

Laptop

👉 Now the object is always created with required data.

What Happens If You Skip Constructor Arguments

$product = new Product();

Output (Error):

Fatal error: Too few arguments to function Product::__construct()

👉 PHP forces you to provide required values — this prevents broken objects.

Default Values in Constructor

class User {
    public $name;
    public $role;

    public function __construct($name, $role = "user") {
        $this->name = $name;
        $this->role = $role;
    }
}

$user = new User("Rohan");

echo $user->role;

Output:

user

What is Inheritance?

Inheritance allows one class to reuse another class’s properties and methods.

👉 You write code once and reuse it in multiple places.

Inheritance Example (Proper Use)

class Animal {
    public function speak() {
        echo "Animal sound";
    }
}

class Dog extends Animal {
    public function bark() {
        echo "Bark";
    }
}

$dog = new Dog();
$dog->speak(); // inherited
$dog->bark();  // own method

Output:

Animal sound
Bark

👉 Dog reuses behavior and adds its own.

Method Overriding

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

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

Output:

Bark

Using Parent Method with parent::

class Dog extends Animal {
    public function speak() {
        parent::speak();
        echo " + Bark";
    }
}

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

Output:

Animal sound + Bark

Constructor in Inheritance (Basic Case)

class Animal {
    public $name;

    public function __construct($name) {
        $this->name = $name;
    }
}

class Dog extends Animal {
}

$dog = new Dog("Tommy");

echo $dog->name;

Output:

Tommy

👉 Child automatically uses parent constructor.

Constructor in Inheritance (Advanced Case)

class Animal {
    public $name;

    public function __construct($name) {
        $this->name = $name;
    }
}

class Dog extends Animal {
    public $breed;

    public function __construct($name, $breed) {
        parent::__construct($name);
        $this->breed = $breed;
    }
}

$dog = new Dog("Tommy", "Labrador");

echo $dog->name . " - " . $dog->breed;

Output:

Tommy - Labrador

👉 Use parent::__construct() when child has extra data.

Common Mistakes Beginners Make

1. Forgetting Constructor Arguments

$product = new Product(); // Error

👉 Constructor requires values.

2. Not Calling Parent Constructor

class Dog extends Animal {
    public function __construct($name) {
        // Missing parent::__construct($name)
    }
}

Correct:

public function __construct($name) {
    parent::__construct($name);
}

3. Thinking Inheritance Copies Code

// Wrong assumption:
class Dog {
    public function speak() {
        echo "Animal sound";
    }
}

👉 In reality, Dog does NOT copy code — it links to Animal.

👉 If you update Animal, all child classes automatically reflect the change.

Practice Exercise

Task 1 (Easy)

Create a User class with constructor.

• Pass name = "Rohan"
• Print the name

Expected Output:

Rohan

Task 2 (Medium)

Create a Product class with constructor (name, price).

• Use: "Laptop", 50000
• Print:

Expected Output:

Laptop - ₹50000

Task 3 (Hard)

Create a Vehicle class:

• start() → prints "Vehicle started"

Create Car class:

• extends Vehicle
• override start() → prints "Car started"

Expected Output:

Car started

Summary

In this lesson, you learned:

• constructors ensure objects are created with required data
• PHP throws an error if constructor arguments are missing
• inheritance allows code reuse across classes
• method overriding changes behavior in child classes
• parent:: lets you reuse parent logic
• parent::__construct() is required when extending constructors