PHP类中动态方法和静态方法调用写法的不解，希望版主大大指导

//error_reporting(0);
class A
{
	public $db='OK';
	private static $_instance;
	public static function getInstance()
    {
		if(!(self::$_instance instanceof self)){
			self::$_instance = new self;
		}
		return self::$_instance;
	}
	//维持find方法中this指针的写法不变
	public function find()
	{
		echo $this->db;
	}
}
class B extends A
{
	static function ex()
	{
		self::getInstance()->find();	
	}
}
$b=new B();
$b->find();
echo "
";
//第二种方法
B::ex();
?>