类的静态方法在继承中的使用，不大明白，self和this怎么转换？

<?phpclass A  //核心{	public $db;	public function __construct(){		$this->db=new B();		}	public function find(){		return  $this->db->b1();		}}class B  //数据库{	public function b1(){		return true;	}	public function b2(){		return false;	}}//**********分隔符***********class C extends A //第三方类{	 static function test(){		return self::find();		}}//当前文件if(C::test()){	echo 'OK';	}?>

class A  //核心{    private static $_Instance;    public $db;    public function __construct(){        $this->db=new B();       }    public function find(){        if(empty(self::$_Instance)) self::$_Instance = new self();        return  self::$_Instance->db->b1();       }}class B  //数据库{    public function b1(){        return true;    }    public function b2(){        return false;    }}//**********分隔符***********class C extends A //第三方类{   static function test(){     return self::find();      }}error_reporting(E_ALL ^ E_STRICT);//当前文件if(C::test()){    echo 'OK';   }