PSR-4 Autoloading Standard
Understand how PSR-4 maps namespace segments to directory paths.
PSR-4 Autoloading Standard is a free PHP Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the PHP Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
What is Autoloading?
Autoloading means PHP automatically loads a class file the first time you use that class — no manual require needed.
Before Autoloading
Without autoloading, every file began with dozens of require_once calls — fragile and hard to maintain.
PSR-4 Rule
PSR-4 maps a namespace prefix to a base directory. Given prefix App\ → src/: App\Models\User loads from src/Models/User.php.
The Mapping Formula
Strip the prefix, replace remaining backslashes with directory separators, append .php.
App\Services\Payment\Stripe → src/Services/Payment/Stripe.php
composer.json autoload Section
Define PSR-4 mappings in composer.json. Run composer dump-autoload after changes.
// composer.json
{
"autoload": {
"psr-4": {
"App\\": "src/"
}
}
}Multiple Namespace Prefixes
A project can have multiple PSR-4 mappings — useful for separating test namespaces.
// composer.json
{
"autoload": { "psr-4": { "App\\": "src/" } },
"autoload-dev": { "psr-4": { "Tests\\": "tests/" } }
}Class Name = File Name
The class name must exactly match the file name (case-sensitive on Linux). class UserService must live in UserService.php.
One Class Per File
PSR-4 requires exactly one class, interface, or trait per file. Multiple definitions break autoloading.
PSR-4 vs PSR-0
PSR-0 (deprecated) used underscores in class names as directory separators. PSR-4 is cleaner and is the current standard.
The Generated Autoloader
After composer dump-autoload, Composer writes vendor/autoload.php. Require it once at the entry point.
<?php
require __DIR__.'/vendor/autoload.php';
$user = new App\Models\User(); // autoloads automaticallyClassmap Fallback
Composer also generates a classmap for files that do not follow PSR-4. Useful for legacy code.
Practical Directory Layout
src/
Models/User.php → App\Models\User
Http/Controllers/
UserController.php → App\Http\Controllers\UserControllerQuick Check
Where does PSR-4 find App\Services\Mailer if prefix App\\ maps to src/?
Frequently asked questions
Is the “PSR-4 Autoloading Standard” lesson free?
Yes — the full text of “PSR-4 Autoloading Standard” is free to read here on the web, and the PHP Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the PHP Academy course, upgrade to CoddyKit PRO.
What will I learn in “PSR-4 Autoloading Standard”?
Understand how PSR-4 maps namespace segments to directory paths. You practise PHP Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start PHP Academy?
No prior experience is required. PHP Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “PSR-4 Autoloading Standard” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this PHP Academy lesson?
Yes. Every PHP Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Declaring and Using Namespaces
- Nested Namespaces and Aliases
- PSR-4 Autoloading Standard
- Registering Autoloaders