class Database{
protected $db;
static function getInstance(){
  if(self::$db){
      return self::$db;
  }else{
   self::$db = new self();
   return self::$db;
  }}

我这么写会提示错误Fatal error: Access to undeclared static property: Imooc\Database::$db in D:\WWW\test_820\Imooc\Database.php on line 21

但是如果我把 $db改成静态的就好了  求原因

static  protected $db;