🔁 PHP Inheritance – Reuse and Extend Functionality with OOP

Learn how to use inheritance in PHP to create parent-child relationships between classes, reuse logic, and extend behavior.

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🧲 Introduction – What Is Inheritance in PHP?

Inheritance is a core concept of Object-Oriented Programming (OOP) that allows one class (called the child or subclass) to inherit properties and methods from another class (called the parent or superclass).

🎯 With inheritance, you can:

• Reuse common logic across multiple classes
• Customize or extend behaviors in subclasses
• Apply the DRY (Don’t Repeat Yourself) principle
• Build hierarchical class structures (e.g., Animal → Dog → Labrador)

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🧱 1. Basic Inheritance Syntax

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

class Dog extends Animal {
}

$dog = new Dog();
$dog->speak(); // Animal sound

➡️ Dog inherits the speak() method from Animal.
➡️ Use the extends keyword to inherit from a base class.

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🎯 2. Overriding Parent Methods

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

$dog = new Dog();
$dog->speak(); // Bark!

➡️ You can override a method in the child class to change its behavior.
➡️ Method names remain the same but are redefined.

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🪜 3. Accessing Parent Methods with parent::

class Cat extends Animal {
    public function speak() {
        parent::speak();
        echo " and Meow!";
    }
}

$cat = new Cat();
$cat->speak(); // Animal sound and Meow!

➡️ Use parent::method() to call the original implementation in the parent class.
➡️ Useful when you want to extend (not replace) the parent behavior.

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🔒 4. Property Inheritance

class Vehicle {
    public $type = "Generic Vehicle";
}

class Car extends Vehicle {
    public function describe() {
        echo "This is a $this->type";
    }
}

$car = new Car();
$car->describe(); // This is a Generic Vehicle

➡️ Properties defined in the parent are accessible in the child class if declared public or protected.

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🚫 5. Final Classes and Methods

final class Base {}

class Child extends Base {} // ❌ Error: Cannot extend final class

class ParentClass {
    final public function greet() {
        echo "Hello!";
    }
}

class ChildClass extends ParentClass {
    // public function greet() {} ❌ Error: Cannot override final method
}

➡️ Use final to prevent inheritance or overriding.
➡️ Ensures certain functionality remains untouched.

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👨‍👩‍👦 6. Multilevel Inheritance

class LivingBeing {
    public function breathe() {
        echo "Breathing...";
    }
}

class Human extends LivingBeing {}

class Developer extends Human {}

$dev = new Developer();
$dev->breathe(); // Breathing...

➡️ A class can inherit from another class that already inherits from another class.
➡️ PHP does not support multiple inheritance (one parent only).

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🧠 Best Practices

• ✅ Use inheritance when classes share common behavior
• ✅ Use protected visibility for reusable internal methods
• ❌ Don’t overuse inheritance — consider composition when appropriate
• ✅ Always call parent::__construct() if the parent has one
• ❌ Avoid deep inheritance trees (more than 2–3 levels)

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📌 Summary – Recap & Next Steps

Inheritance lets you build hierarchical class relationships, reuse functionality, and avoid repetitive code. It’s one of the most powerful tools in PHP OOP — when used carefully.

🔍 Key Takeaways:

• Use extends to inherit from a parent class
• Override methods for customized behavior
• Use parent:: to call the parent method
• final blocks further inheritance or overriding

⚙️ Real-World Use Cases:
User → Admin → SuperAdmin, Animal → Dog → Bulldog, Controller → ApiController, Vehicle → Car → ElectricCar

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❓ Frequently Asked Questions (FAQs)

❓ Can a PHP class extend multiple classes?
❌ No. PHP supports only single inheritance.

❓ Can a child class override a parent constructor?
✅ Yes, but you can also call parent::__construct() inside it.

❓ What’s the difference between public, private, and protected in inheritance?
✅ public and protected properties/methods are inherited. private is not.

❓ When should I use final?
✅ Use it to lock down classes or methods you don’t want altered.

❓ Is inheritance better than composition?
🔸 Use inheritance for “is-a” relationships; use composition for “has-a” relationships.

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