Autoloading PHP Classes

When creating multiple classes you will usually create them in separate files for readability and easier code maintenance. To use these classes in your code you must require the class files:


require __DIR__.'/../src/Auth.php';
require __DIR__.'/../src/Config.php';

$auth = new Auth();
$config = new Config();
// ...

The list of required files can soon become very long when dealing with a lot of classes. PHP offers classes autoloading, for example, the spl_autoload_register() function simplifies autoloading:

// loader.php

spl_autoload_register(function ($class) {
    include __DIR__.'/src/'.$class.'.php';

PSR-4 and Composer

How to autoload classes is a matter of preference and depends on the developer. In case of open source ecosystem libraries, frameworks, and other people’s code this soon becomes an issue.

Therefore there has been an initiative from PHP-FIG to standardize the autoloading with the PSR-4 autoloading standard, which became the de-facto standard for autoloading in PHP and specially with the rise of Composer, the PHP package manager.

Autoloading classes with Composer is very simple and advanced enough to meet any requirement you’ll meet in your applications.

To use the Composer, create a composer.json file in the root of your project with composer init command which generates the initial composer.json file you can use and extend further on. Our example will use the following:

    "name": "vendor/example-project",
    "description": "Example project",
    "type": "project",
    "require": {
        "php": "^7.1",
        "monolog/monolog": "^1.21"
    "require-dev": {
        "phpunit/phpunit": "^5.6"
    "autoload": {
        "psr-4": {"Vendor\\ExampleProject\\": "src/"}
    "autoload-dev": {
        "psr-4": { "Vendor\\ExampleProject\\Tests\\": "tests/" }

The most common file structure for PHP can be the following:


The src folder contains your application classes, the vendor folder is created automatically by Composer when installing 3rd party packages or a framework. The composer.lock file is automatically handled by Composer and contains specific versions of your installed 3rd party packages. The tests folder can contain the unit and functional tests.

The public/index.php file also known as the front controller can be one of the places where you could use the Composer’s autoloader:


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

use Acme\ExampleProject\App;

$app = new App();


The predecessor of PSR-4 autoloading standard was the PSR-0, which is now deprecated and shouldn’t be used anymore. Some libraries might still use it and Composer offers that also. For creating new code, stick to PSR-4 autoloading standard and make your development life easier and more consistent.

See Also

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Content of this work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0) license. Code snippets in examples are published under the CC0 1.0 Universal (CC0 1.0). Thanks to all the contributors.