将$ db对象传递给其他类,以便它们可以访问数据库

   private $db;

   public function __construct($db) {

       $this->db = $db;

   }

   public function doSomething() {

       // Access the database using $this->db

   }

   private static $instance;

   private function __construct($db) {

       self::$instance = $db;

   }

   public static function getInstance() {

       if (self::$instance === null) {

           self::$instance = new Database($db);

       }

       return self::$instance;

   }

   public function doSomething() {

       $db = Database::getInstance();

       // Access the database using $db

   }

   private $db;

   public function __construct($db) {

       $this->db = $db;

   }

   public function getDb() {

       return $this->db;

   }

   private $serviceLocator;

   public function __construct($serviceLocator) {

       $this->serviceLocator = $serviceLocator;

   }

   public function doSomething() {

       $db = $this->serviceLocator->getDb();

       // Access the database using $db

   }