Autoloading

Quick Introduction

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

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

Without understanding Autoloading, 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

• Autoloading 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 autoloading 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 Autoloading. 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);

// PSR-4 autoloading via Composer (see composer.json "autoload" section)
/*
"autoload": {
  "psr-4": {
    "Acme\\Shop\\": "src/"
  }
}
*/
require __DIR__ . '/vendor/autoload.php';

use Acme\Shop\Cart\CartService;

$cart = new CartService();
$cart->add('SKU-100', qty: 2);
$cart->add('SKU-200', qty: 1);

printf("Cart items: %d | Total: $%.2f\n", $cart->count(), $cart->total());
spl_autoload_register(function (string $class): void {
    // Composer handles PSR-4; custom autoloaders register afterward
});

Line by line

1. — part of the autoloading example; run the file to see the result.
2. declare(strict_types=1); — turns on strict type checking for this file.
3. /* — part of the autoloading example; run the file to see the result.
4. "autoload": { — part of the autoloading example; run the file to see the result.
5. "psr-4": { — part of the autoloading example; run the file to see the result.

User #42 (user@example.com) balance $19.99

Real-World Example

In the REST API, autoloading 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: Autoloading exercise for REST API