PHP Abstract Classes vs Interfaces: Know the Difference

What is it?

An abstract class is a partially-built class — it can have shared code AND require child classes to implement certain methods. An interface is a pure contract — it defines what methods must exist but provides zero code.

Why does it matter?

Choosing wrong creates fragile designs. Use the wrong one and you end up with duplicate code or impossible-to-reuse structures. This is one of the most commonly asked OOP interview questions at TCS, Infosys, and Google.

Learn abstract classes vs interfaces with real-world examples and guidelines.

Real-World Use Cases

🔷 Shape hierarchy - Shape abstract class holds color property and describe() method. Circle, Square, Triangle each implement the abstract area() method differently.
📤 Export contracts - Exportable interface ensures every report type has exportToCsv() and exportToJson(). The interface itself has no code.
🗄️ Database drivers - AbstractDatabase holds shared connection logic. MySQLDriver and PostgreSQLDriver each implement the abstract query() method.
🔔 Notification channels - Notifiable interface with send(). SMS, Email, Slack classes all implement it — completely unrelated classes, same contract.

Abstract Class — Partial Blueprint

Use when:

✅ Classes share common properties or code (IS-A relationship)

✅ You want to provide a partial implementation

✅ Circle IS-A Shape, Dog IS-AN Animal

abstract class Animal {
    public function eat() {
        echo "Eating...";
    }

    abstract public function sound();
}

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

Interface — Pure Contract

Use when:

✅ Defining a capability/contract (CAN-DO relationship)

✅ Unrelated classes need the same method signatures

✅ You need multiple "parents" (a class can implement many interfaces)

✅ Logger, Printable, Exportable — cross-cutting concerns

interface Payment {
    public function pay($amount);
}

class PayPal implements Payment {
    public function pay($amount) {
        echo "Paid $amount via PayPal";
    }
}

Decision Guide

Requirement	Use
Share common code	Abstract Class
Define only rules/contract	Interface
Need properties	Abstract Class
Need method implementation	Abstract Class
Multiple inheritance-like behavior	Interface
Unrelated classes follow same contract	Interface

Q: Can an abstract class implement an interface?

Yes. An abstract class can implement an interface without providing all methods — leaving them abstract for child classes. Useful for sharing common code among classes that satisfy the same contract.

Quick Access

Abstract Class — Partial Blueprint

Interface — Pure Contract

Decision Guide

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