SOLID Principles

Quick Introduction

SOLID Principles is a single PHP idea you'll use in almost every backend project.

In this lesson you learn only solid principles — 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 solid principles working correctly before storing data or showing a dashboard.

Without understanding SOLID Principles, 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

• SOLID Principles 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 solid principles 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 SOLID Principles. 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 Billing;

// Single Responsibility + Dependency Inversion
interface Notifier { public function notify(string $message): void; }

final class EmailNotifier implements Notifier {
    public function notify(string $message): void { error_log("EMAIL: {$message}"); }
}

final class InvoiceService {
    public function __construct(private Notifier $notifier) {}

    public function sendInvoice(int $invoiceId): void {
        $this->notifier->notify("Invoice #{$invoiceId} ready");
    }
}

// Open/Closed: add SlackNotifier without changing InvoiceService
(new InvoiceService(new EmailNotifier()))->sendInvoice(9001);

Line by line

1. — part of the solid principles example; run the file to see the result.
2. declare(strict_types=1); — turns on strict type checking for this file.
3. namespace Billing; — part of the solid principles example; run the file to see the result.
4. interface Notifier { public function notify(string $message): void; } — part of the solid principles example; run the file to see the result.
5. final class EmailNotifier implements Notifier { — part of the solid principles example; run the file to see the result.

{"status":"ok"}

Real-World Example

In the E-Commerce Backend, solid principles 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: SOLID Principles exercise for E-Commerce Backend