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docs/guide/basic-autoloading.md

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+Autoloading
+===========
+
+> Note: This chapter is under development.
+
+All classes, interfaces and traits are loaded automatically at the moment they are used. There's no need to use `include` or `require`. It is true for Composer-loaded packages as well as Yii extensions.
+
+Yii's autoloader works according to [PSR-4](https://github.com/php-fig/fig-standards/blob/master/proposed/psr-4-autoloader/psr-4-autoloader.md).
+That means namespaces, classes, interfaces and traits must correspond to file system paths and file names accordinly, except for root namespace paths that are defined by an alias.
+
+For example, if the standard alias `@app` refers to `/var/www/example.com/` then `\app\models\User` will be loaded from `/var/www/example.com/models/User.php`.
+
+Custom aliases may be added using the following code:
+
+```php
+Yii::setAlias('@shared', realpath('~/src/shared'));
+```
+
+Additional autoloaders may be registered using PHP's standard `spl_autoload_register`.