How Composer Autoloading Works in PHP

Composer's autoloader lets you use any class in your project without a manual require or include statement, by mapping namespaces to directories as declared in composer.json.

"autoload": {
  "psr-4": {
    "App\\": "app/"
  }
}

Steps

1. Declare a PSR-4 mapping in composer.json's autoload.psr-4 section, pairing a namespace prefix with a folder
2. Require vendor/autoload.php once at your application's entry point
3. Composer generates a class map that translates 'App\Models\User' into app/Models/User.php automatically when that class is first referenced

How it works

PSR-4 is a PHP-FIG standard specifying that a namespace segment corresponds directly to a directory segment, and the class name matches the file name. Composer reads this mapping and registers an autoload function via spl_autoload_register().

Things to watch for

• After adding a new class or changing composer.json's autoload section, run 'composer dump-autoload' to regenerate the autoloader's internal class map
• In production, run 'composer dump-autoload -o' (optimized) to generate a flat class map for faster lookups instead of PSR-4's directory-scanning fallback

FAQ

Why isn't my new class being found by Composer?

Run composer dump-autoload — Composer sometimes needs to regenerate its optimized class map after new files are added, especially if you're using the optimized (-o) autoloader.