retrologin.php          # Main plugin file (header only)
app/
  Admin/
    Settings.php        # Class: Retrologin\Admin\Settings
  Frontend/
    Login.php           # Class: Retrologin\Frontend\Login
inc/
  bootstrap/
    app.php             # Bootstrap file

<?php

declare(strict_types=1);

/**
 * Short description of the class.
 *
 * Longer description if needed.
 *
 * @package Retrologin
 * @since   0.1.0
 */

namespace Retrologin\Admin;

if (! defined('ABSPATH')) {
    exit;
}

/**
 * Class description.
 *
 * @since 0.1.0
 */
class Settings {

    /**
     * Constructor.
     *
     * @since 0.1.0
     */
    public function __construct() {
        // Hooks, filters, etc.
    }

    /**
     * Method description.
     *
     * @param string $param Description.
     * @return void
     * @since 0.1.0
     */
    public function method( string $param ): void {
        // ...
    }
}