psr4_class_loader.rst

.. index::
   single: ClassLoader; PSR-4 Class Loader

The PSR-4 Class Loader

Libraries that follow the PSR-4 standard can be loaded with the Psr4ClassLoader.

Note

If you manage your dependencies via Composer, you get a PSR-4 compatible autoloader out of the box. Use this loader in environments where Composer is not available.

Tip

All Symfony components follow PSR-4.

Usage

The following example demonstrates how you can use the :class:`Symfony\\Component\\ClassLoader\\Psr4ClassLoader` autoloader to use Symfony's Yaml component. Imagine, you downloaded both the ClassLoader and Yaml component as ZIP packages and unpacked them to a libs directory. The directory structure will look like this:

libs/
    ClassLoader/
        Psr4ClassLoader.php
        ...
    Yaml/
        Yaml.php
        ...
config.yml
demo.php

In demo.php you are going to parse the config.yml file. To do that, you first need to configure the Psr4ClassLoader:

use Symfony\Component\ClassLoader\Psr4ClassLoader;
use Symfony\Component\Yaml\Yaml;

require __DIR__.'/lib/ClassLoader/Psr4ClassLoader.php';

$loader = new Psr4ClassLoader();
$loader->addPrefix('Symfony\Component\Yaml\\', __DIR__.'/lib/Yaml');
$loader->register();

$data = Yaml::parse(file_get_contents(__DIR__.'/config.yml'));

First of all, the class loader is loaded manually using a require statement, since there is no autoload mechanism yet. With the :method:`Symfony\\Component\\ClassLoader\\Psr4ClassLoader::addPrefix` call, you tell the class loader where to look for classes with the Symfony\Component\Yaml\ namespace prefix. After registering the autoloader, the Yaml component is ready to be used.