Autoloading Your Own Code

The earlier Autoloading page showed how vendor/autoload.php automatically loads classes from packages you install. That same autoloader can also load your own classes — the code you write for your project — with zero manual include or require statements, using the autoload section of composer.json.

The autoload Section and PSR-4

PSR-4 is a PHP standard that maps namespace prefixes to folders on disk. Composer understands PSR-4 natively: you tell it "this namespace prefix lives in this folder", and it generates the mapping code for you.

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

This tells Composer: "any class inside the App namespace lives inside the src/ folder." Note that JSON requires a backslash to be written as two backslashes inside a string, so "App\\" represents the single namespace separator App\.

A Worked Example

With the mapping above in place, a class named App\Greeter must live at src/Greeter.php. The folder path after the namespace prefix mirrors the rest of the namespace, and the file name matches the class name.

<?php
namespace App;

class Greeter
{
    public function greet($name)
    {
        return "Hello, $name!";
    }
}

Once vendor/autoload.php has been required, App\Greeter can be used anywhere in the project with no manual require for src/Greeter.php at all.

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

$greeter = new \App\Greeter();
echo $greeter->greet('World'); // Hello, World!

The Gotcha: Run composer dump-autoload

Composer generates the actual autoloader files once, based on whatever the autoload section says at that moment. If you add the autoload block for the first time, change the namespace mapping, or simply create a brand new class file, Composer doesn't notice automatically — you have to tell it to regenerate the mapping.

composer dump-autoload

You'll also see Composer run this step automatically after composer install or composer require — but any time you edit the autoload section by hand, or add files in a way Composer wouldn't otherwise notice, run it yourself to be safe.