PHP Composer

What is Composer?

Composer is PHP's package manager. It downloads libraries from Packagist and generates an autoloader so classes load automatically when you use them.

Getting started

composer init                        # create composer.json
composer require guzzlehttp/guzzle   # add a package
composer require --dev phpunit/phpunit
composer install                     # install from composer.lock

Commit composer.json and composer.lock; never commit the vendor/ folder.

Autoloading your own code (PSR-4)

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

Run composer dump-autoload, then include one file and every class in src/ loads on demand:

<?php
require __DIR__ . "/vendor/autoload.php";

use App\Models\User;      // loads src/Models/User.php automatically
$user = new User("Ada");

How autoloading works

Composer registers a function with spl_autoload_register(); PHP calls it the first time an unknown class is used:

<?php
spl_autoload_register(function (string $class) {
    echo "autoloader asked for: $class\n";
    // A real autoloader would require src/Models/User.php here.
    if ($class === "App\\Models\\User") {
        eval('namespace App\Models; class User { public function __construct(public string $name) {} }');
    }
});

$user = new App\Models\User("Ada");
echo "created ", $user->name, "\n";
$again = new App\Models\User("Lin");   // already loaded: no second call
echo "created ", $again->name, "\n";

autoloader asked for: App\Models\User
created Ada
created Lin