plugin-autoload.php

<?php

public function boot(PluginManager $plugins)
{
    // store paths of class files of plugins
    $src_paths = [];

    foreach ($plugins->getPlugins() as $plugin) {
        $src_paths[$plugin->getNameSpace()] = $plugin->getPath()."/src";
    }

    $this->registerClassAutoloader($src_paths);

    // call bootstrappers here
}

/**
 * Register class autoloader for plugins.
 *
 * @return void
 */
protected function registerClassAutoloader($paths)
{
    spl_autoload_register(function ($class) use ($paths) {
        // traverse in registered plugin paths
        foreach ((array) array_keys($paths) as $namespace) {
            if ($namespace != '' && mb_strpos($class, $namespace) === 0) {
                // parse real file path
                $path = $paths[$namespace].Str::replaceFirst($namespace, '', $class).".php";
                $path = str_replace('\\', '/', $path);

                if (file_exists($path)) {
                    // include class file if it exists
                    include $path;
                }
            }
        }
    });
}