PHP design pattern Singleton (single case mode)

Copy code The code is as follows:
/**
* Singleton pattern
*
* Ensures that a class has only one instance and provides a global access point to access it
*
*/
class Singleton
{
static private $_instance = null;

private function __construct()
{
}

static public function getInstance ()
{
if(is_null(self::$_instance)) {
self::$_instance = new Singleton();
}
return self::$_instance;
}

public function display()
{
echo "it is a singlton class function";
}
}

// $obj = new Singleton(); // Declaration cannot be successful
$obj = Singleton::getInstance();
var_dump($obj);
$obj->display();

$obj1 = Singleton::getInstance();
var_dump(($obj === $obj1));