Interfaces, Abstract Classes & Polymorphism

In Plain English

Build loosely-coupled, extensible architectures. Learn pure contract interfaces (interface, implements), partial implementations with abstract classes, multiple interface inheritance, and polymorphic dependency injection.

The Mental Model

"An interface is like a universal electrical wall outlet standard: any lamp, laptop charger, or blender that matches the 3-prong interface can plug in and draw power without caring how the power company generates the electricity."

Deep Dive: How It Works

Interfaces: Pure contracts declaring method signatures without implementations (e.g. interface Logger { public function log(string $msg): void; }).

Abstract Classes: Classes that cannot be instantiated directly and can contain both concrete methods and abstract method signatures.

Multiple Interfaces: A single class can implement multiple interfaces (class Repository implements Reader, Writer).

Polymorphism: Functions accept interface type hints, allowing arbitrary implementations to be passed interchangeable.

Syntax Blueprint

<?php

interface PaymentGateway {
    public function charge(float $amount): bool;
}

class StripeGateway implements PaymentGateway {
    public function charge(float $amount): bool { return true; }
}

Define contract with interface and implement all methods in concrete classes with implements.

Core Rules to Remember

Contract Enactment: PHP enforces that implementing classes must define all interface methods with matching signatures.

Dependency Inversion: High-level modules should depend on interfaces, not concrete classes.

Interface Constants: Interfaces can declare public constants accessible on the interface or implementing classes.

Common Beginner Traps & How to Fix Them

Mistake:Declaring non-public methods in an interface.

Why it happens: Interface methods are contracts by definition and must always be public.

How to fix: All methods declared inside an interface must be public.

Pro Tip: Keep interfaces focused and granular (Interface Segregation Principle): favor multiple small interfaces over one massive interface.

Live Interactive Example

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Polymorphic Payment Gateways

PHP 8.2 / CLI

Output Console

Click "Run Code" to view the rendered output.

How it works: checkout() polymorphically executed both gateways through the PaymentGateway interface contract.

Your Turn: Micro Challenge

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

Micro Exercise

Implement Renderable Interface

Define an interface Renderable with public function render(): string;.
Create a class Button implements Renderable with public function __construct(public string $label) {}.
Implement render() returning "<button>{$this->label}</button>".
In main, create $btn = new Button("Submit") and print $btn->render().

Sandbox Output

Click "Run & Check" to test your solution.

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