Composer and autoloading

Last reviewed Jun 1, 2026 Content v20260601

Track mode: server_script
Means: Server runner
Reading: ~1 min
Level: intermediate

This lesson

This lesson teaches Composer and autoloading: the syntax, APIs, and habits you need before advancing in PHP.

Composer autoloading is how real PHP projects organize code—without it, namespaces feel academic.

You will apply Composer and autoloading in contexts like: PSR-4 autoloaded apps, Laravel packages, and shared internal libraries.

Write PHP in the editor and click Run on server—the dev runner executes your script and returns stdout/stderr (set LEARNING_RUNNER_ENABLED=true locally).

Toward the end of the track—consolidate before capstone-style review lessons.

Composer is PHP's dependency manager. It installs packages from Packagist and generates an autoloader so you use App\Models\User; instead of manual require chains.

composer.json essentials

{
  "require": {
    "monolog/monolog": "^3.0"
  },
  "autoload": {
    "psr-4": {
      "App\\": "src/"
    }
  }
}

Workflow

composer install — install from lock file
composer require vendor/package — add dependency
require 'vendor/autoload.php'; in front controller

PSR-4 autoloading

Namespace App\Controllers\UserController maps to src/Controllers/UserController.php. Frameworks (Laravel, Symfony) build on this convention.

Important interview questions and answers

Q: composer.json vs composer.lock?
A: JSON declares ranges; lock pins exact versions for reproducible installs—commit lock in apps.
Q: PSR-4?
A: Autoloading standard mapping namespaces to directory paths.

Self-check

Which file do you require to enable autoloading?
Why commit composer.lock?

Tip: Commit composer.lock in applications so deploys install identical dependency versions.

Interview prep

PSR-4 autoloading?: Maps namespace prefixes to directory paths so use App\Model\User loads the correct file without manual requires.

Interview tip Lesson completion confidence

Can you explain this lesson in 30 seconds without reading notes?

Self-reflection (saved on this device)

Not saved yet.

Playground

Runs on the configured server runner (dev: npm run runner with LEARNING_RUNNER_ENABLED=true). Output appears below the editor.

Code runner not available

Server runner is disabled. Set LEARNING_RUNNER_ENABLED=true and LEARNING_RUNNER_URL in .env (see .env.example).

Check yourself

Multiple choice — immediate feedback.

Discussion

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Starter discussion topics

PSR-4 mapping?
vendor commit policy?

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