Autoloading with Composer

After Composer installs packages, it creates one file that makes every class available: vendor/autoload.php. You include it once.

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

// every installed package, and your own classes, are now autoloaded

Put this line at the start of your front controller (usually public/index.php), and in command-line scripts. Use __DIR__ so the path is always right (see File Paths).

What the autoloader does

It registers one spl_autoload_register() function (see Autoloading). When your code uses a class, Composer’s autoloader finds the file from a map it generated when you ran composer install.

Autoload your own classes

Add an autoload section to composer.json:

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

Then regenerate the autoloader:

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composer dump-autoload

Now App\Service\Greeter is loaded from src/Service/Greeter.php automatically. PSR-4 Autoloading with Composer goes deeper.

When to run dump-autoload

• After you change the autoload section of composer.json.
• After you add a class to a folder that is mapped with a classmap (not needed for PSR-4, where new files are found automatically).

For PSR-4, adding a new class file in the right folder with the right namespace is enough. You do not need to run anything.

Other autoload types

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{
    "autoload": {
        "psr-4": { "App\\": "src/" },
        "files": ["src/helpers.php"],
        "classmap": ["legacy/"]
    }
}

Optimised autoloader for production

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composer install --no-dev --optimize-autoloader

or

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composer dump-autoload --optimize

This builds a complete class map, so the autoloader does not search the file system. It is faster on a live server. In Composer’s own output, you will see a line such as “Generated optimized autoload files containing 397 classes”.

What lives in vendor/composer

vendor/composer/ holds the generated autoload files (autoload_psr4.php, autoload_classmap.php…). They are generated, so never edit them. They are rebuilt every time.

Common error: class not found

Class "App\Service\Greeter" not found usually means one of these:

1. vendor/autoload.php was never required.
2. The namespace in the file does not match the folder (check capitals).
3. The autoload section is missing or wrong in composer.json.
4. You changed composer.json and did not run composer dump-autoload.