intermediate 14 min read

Interfaces & Abstract Classes

Contracts with interfaces, abstract classes, and when to use each.

Interfaces

interface Logger {
    public function log(string \$message): void;
    public function error(string \$message): void;
}

class FileLogger implements Logger {
    public function log(string \$message): void {
        // write to file
    }
    public function error(string \$message): void {
        \$this->log("[ERROR] \$message");
    }
}

Abstract Classes

abstract class Database {
    abstract public function connect(): void;
    public function query(string \$sql): array {
        \$this->connect();
        // execute query
    }
}

Interface vs Abstract

Interface: contract only (no implementation), multiple inheritance
Abstract: can have implementation, single inheritance

Multiple interface implementation

A class can implement multiple interfaces.

Examples

<?php
interface CanFly { public function fly(): string; }
interface CanSwim { public function swim(): string; }
class Duck implements CanFly, CanSwim {
    public function fly(): string { return 'Duck flying'; }
    public function swim(): string { return 'Duck swimming'; }
}
echo (new Duck)->fly() . ' ' . (new Duck)->swim();

Table of Contents

Related Lessons

Interfaces & Abstract Classes

Interfaces

Abstract Classes

Interface vs Abstract

Multiple interface implementation

Examples

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