Inheritance, Access Modifiers & Method Overriding

In Plain English

Organize object hierarchies through inheritance. Master encapsulation with access modifiers (public, protected, private), extend base classes with extends, invoke parent methods with parent::, and prevent extension using the final keyword.

The Mental Model

"Access modifiers are like security clearances in a corporate headquarters: public is the open lobby, protected is the employee-only workspace shared with sister branches (subclasses), and private is the CEO personal locked vault."

Deep Dive: How It Works

Access Modifiers: public (everywhere), protected (class & subclasses only), private (declaring class only).

Inheritance: class Manager extends Employee inherits all non-private methods and properties.

Parent Invocation: parent::__construct(...) or parent::doWork() calls overridden base implementations.

Final Classes & Methods: final prevents other classes from extending or overriding critical security routines.

Syntax Blueprint

<?php

class BaseAccount {
    protected float $balance = 0;
}

class SavingsAccount extends BaseAccount {
    public function addInterest() { $this->balance *= 1.05; }
}

Use protected for shared subclass state and extends to inherit base class behavior.

Core Rules to Remember

Encapsulation: Keep internal state private or protected, exposing only intentional public API methods.

Method Overriding: Subclasses can override parent methods to specialize behavior while calling parent:: when needed.

Liskov Substitution: Subclasses must maintain compatible parameter and return types with the parent signature.

Common Beginner Traps & How to Fix Them

Mistake:Trying to access private properties from a subclass ($this->privateField).

Why it happens: private properties are strictly restricted to the declaring class.

How to fix: Change modifier to protected if subclasses legitimately need access.

Pro Tip: Mark utility classes and value objects as final class to enforce composition over fragile deep inheritance trees.

Live Interactive Example

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Inheritance and Parent Constructor Chaining

PHP 8.2 / CLI

Output Console

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How it works: Manager inherited getDetails() and accessed protected $salary in getTotalComp().

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Extend Base Notification Class

Create a class Notification with public function send(string $to): string returning "Notification sent to {$to}".
Create a subclass EmailNotification extends Notification that overrides send(string $to): string to return "Email dispatched to {$to}".
In main script, create $email = new EmailNotification() and print the result of $email->send("[email protected]").

Sandbox Output

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