How to set up automatic class loading using Composer?

Class autoloading with Composer is configured in the composer.json file. Composer supports two main types of autoloading: psr-4 and psr-0, as well as file autoloading (files) and classmap definition.

The most modern and recommended method is psr-4. It links a namespace prefix to a directory.

{
    "autoload": {
        "psr-4": {
            "MyNamespace\\": "src/",
            "AnotherNamespace\\": "lib/"
        },
        "files": [
            "config/helpers.php"
        ],
        "classmap": [
            "database/migrations",
            "database/seeds"
        ]
    }
}

• psr-4: For classes starting with MyNamespace\, Composer will look for them in the src/ directory. For classes with the prefix AnotherNamespace\ – in lib/.
• files: Allows loading specified files on each request. Often used for non-class functions or constants.
• classmap: Scans specified directories or files, finds all classes, and builds a static map of their paths. Efficient for libraries without explicit namespaces or with non-standard structures.

After making changes to composer.json, you need to run the command to generate or update autoload files:

composer dump-autoload

This command creates or updates the vendor/autoload.php file, which is then included in the initial script execution point (index.php or similar):

<?php

require __DIR__ . '/vendor/autoload.php';

use MyNamespace\MyClass;
use AnotherNamespace\Utility;

$obj1 = new MyClass();
$obj2 = new Utility();

?>

Composer automatically creates the spl_autoload_register function, which will attempt to load a class when needed, using the information from the generated autoload files.