framework/BaseYii.php
public static function autoload($className)
    {
        if (isset(static::$classMap[$className])) {
            $classFile = static::$classMap[$className];
            if ($classFile[0] === '@') {
                $classFile = static::getAlias($classFile);
            }
        } elseif (strpos($className, '\\') !== false) {
            $classFile = static::getAlias('@' . str_replace('\\', '/', $className) . '.php', false);
            if ($classFile === false || !is_file($classFile)) {
                return;
            }
        } else {
            return;
        }

        include $classFile;

        if (YII_DEBUG && !class_exists($className, false) && !interface_exists($className, false) && !trait_exists($className, false)) {
            throw new UnknownClassException("Unable to find '$className' in file: $classFile. Namespace missing?");
        }
    }
public static function autoload($className)
    {
        if (isset(static::$classMap[$className])) {
            $classFile = static::$classMap[$className];
            if ($classFile[0] === '@') {
                $classFile = static::getAlias($classFile);
            }
        } elseif (strpos($className, '\\') !== false) {
            $classFile = static::getAlias('@' . str_replace('\\', '/', $className) . '.php', false);
            if ($classFile === false || !is_file($classFile)) {
                return;
            }
        } else {
            return;
        }

        include $classFile;

        if (YII_DEBUG && !class_exists($className, false) && !interface_exists($className, false) && !trait_exists($className, false)) {
            throw new UnknownClassException("Unable to find '$className' in file: $classFile. Namespace missing?");
        }
    }
Class autoload loader.

This method is invoked automatically when PHP sees an unknown class.
The method will attempt to include the class file according to the following procedure:

1. Search in [[classMap]];
2. If the class is namespaced (e.g. `yii\base\Component`), it will attempt
to include the file associated with the corresponding path alias
(e.g. `@yii/base/Component.php`);

This autoloader allows loading classes that follow the [PSR-4 standard](http://www.php-fig.org/psr/psr-4/)
and have its top-level namespace or sub-namespaces defined as path aliases.

Example: When aliases `@yii` and `@yii/bootstrap` are defined, classes in the `yii\bootstrap` namespace
will be loaded using the `@yii/bootstrap` alias which points to the directory where bootstrap extension
files are installed and all classes from other `yii` namespaces will be loaded from the yii framework directory.

Also the [guide section on autoloading](guide:concept-autoloading).

@param string $className the fully qualified class name without a leading backslash "\"
@throws UnknownClassException if the class does not exist in the class file