What can be improved/fixed in the class (Singleton)?

Question

PHP

I finally got down to fundamental things in order to tighten up the theoretical base.
I am currently reading a book by Matt Zandstra - PHP: Objects, Patterns and Programming Techniques.
I got to the chapter with patterns and I'm trying it in practice I
wrote a class to get the configuration using Singleton,
please tell me what jambs are there? What could be improved/fixed

<?php

namespace App\Core;

use Symfony\Component\Dotenv\Dotenv;

/**
 * Class Config
 * @package App\Core
 */
class Config {

  /**
   * @var  static
   */
  private static $instance;

  /**
   * @var array
   */
  private $params = [];

  /**
   * Config constructor.
   */
  private function __construct() {
    $dotenv = new Dotenv();
    // здесь немного смущает путь к конфигурационному файлу
    $dotenv->load( __DIR__ . '/../../.env' );
    $this->params = $_ENV;
  }

  /**
   * @return static
   */
  public static function getInstance(): self {
    // в этом методе можно использовать и static и self Есть ли какая то разница
    if ( empty( self::$instance ) ) {
      return new Config();
    }

    return self::$instance;
  }

  /**
   * @return array
   */
  public function getParams(): array {
    return $this->params;
  }

  /**
   * @param string $key
   *
   * @return string
   */
  public function getParam( string $key ): string {
    return $this->params[ $key ];
  }
}

Answer 1

public static function getInstance(): self {
    if ( empty( self::$instance ) ) {
      self::$instance = new Config();
    }

    return self::$instance;
  }

Answer 2

You can also ask the class https://refactoring.guru/en/design-patterns/singleton