Dependency Injection

Quick Introduction

Dependency Injection is a single PHP idea you'll use in almost every backend project.

In this lesson you learn only dependency injection — 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 REST API. After a user signs in, you need dependency injection working correctly before storing data or showing a dashboard.

Without understanding Dependency Injection, 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

• Dependency Injection 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 (REST API) uses dependency injection 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 Dependency Injection. 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 App\Container;

interface Mailer { public function send(string $to, string $subject, string $body): void; }

final class SmtpMailer implements Mailer {
    public function send(string $to, string $subject, string $body): void {
        error_log("SMTP to={$to} subject={$subject}");
    }
}

final class WelcomeService {
    public function __construct(private Mailer $mailer) {}

    public function onboard(string $email): void {
        $this->mailer->send($email, 'Welcome', 'Thanks for joining!');
    }
}

// Manual DI — frameworks use containers (Laravel, Symfony)
$service = new WelcomeService(new SmtpMailer());
$service->onboard('user@example.com');

Line by line

1. — part of the dependency injection example; run the file to see the result.
2. declare(strict_types=1); — turns on strict type checking for this file.
3. namespace App\Container; — part of the dependency injection example; run the file to see the result.
4. interface Mailer { public function send(string $to, string $subject, string $body): void; } — part of the dependency injection example; run the file to see the result.
5. final class SmtpMailer implements Mailer { — part of the dependency injection example; run the file to see the result.

{"status":"ok"}

Real-World Example

In the REST API, dependency injection 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: Dependency Injection exercise for REST API