Design Patterns

Quick Introduction

Design Patterns is a single PHP idea you'll use in almost every backend project.

In this lesson you learn only design patterns — not five topics at once. By the end you can write a small working example and explain it in an interview.

We connect each lesson to our course projects: Login System, Blog CMS, REST API, Inventory, Employee Management, and E-Commerce Backend.

Business Problem

You're building the E-Commerce Backend. After a user signs in, you need design patterns working correctly before storing data or showing a dashboard.

Without understanding Design Patterns, the team ships bugs: wrong totals, broken sessions, or type errors that only appear in production. This lesson fixes that with one clear pattern you can copy into your project today.

Core Concept

• Design Patterns is one focused idea — learn it before mixing with other PHP topics.
• Use it in PHP 8.3+ with declare(strict_types=1); at the top of every file.
• Our course project (E-Commerce Backend) uses design patterns in real handlers.
• Run small scripts with php file.php after each change — don't just read.
• Interviewers ask for a one-minute explanation plus a tiny code sample.

Syntax

Core syntax for Design Patterns. Every keyword below appears in production PHP — Laravel and Symfony use the same primitives under the hood.

declare(strict_types=1);

Keywords: declare · strict_types · namespace

Step-by-Step Example

Run this script locally. Change one value, run again, and watch what changes.

<?php
declare(strict_types=1);

namespace Patterns;

interface Logger { public function log(string $msg): void; }

final class FileLogger implements Logger {
    public function log(string $msg): void { file_put_contents('app.log', $msg . PHP_EOL, FILE_APPEND); }
}

// Singleton (use sparingly — prefer DI container)
final class Log {
    private static ?Logger $instance = null;
    public static function get(): Logger {
        return self::$instance ??= new FileLogger();
    }
}

Log::get()->log('Order placed');

Line by line

1. — part of the design patterns example; run the file to see the result.
2. declare(strict_types=1); — turns on strict type checking for this file.
3. namespace Patterns; — part of the design patterns example; run the file to see the result.
4. interface Logger { public function log(string $msg): void; } — part of the design patterns example; run the file to see the result.
5. final class FileLogger implements Logger { — part of the design patterns example; run the file to see the result.

Design Patterns OK

Real-World Example

In the E-Commerce Backend, design patterns appears in a single request handler — not spread across ten files. Keep the example small, test it with php, then paste the pattern into your project branch.

<?php
declare(strict_types=1);

// TODO: Design Patterns exercise for E-Commerce Backend