被这个PHP异常混淆了try..catch嵌套

class MyException extends Exception {}
class AnotherException extends MyException {}

class Foo {
  public function something() {
    print "throwing AnotherExceptionn";
    throw new AnotherException();
  }
  public function somethingElse() {
    print "throwing MyExceptionn";
    throw new MyException();
  }
}

$a = new Foo();

try {
  try {
    $a->something();    

  } catch(AnotherException $e) {
    print "caught AnotherExceptionn";
    $a->somethingElse();    
  } catch(MyException $e) {
    print "caught MyExceptionn";
  }
} catch(Exception $e) {
  print "caught Exceptionn";
}

我会期望这样输出：

throwing AnotherException
caught AnotherException
throwing MyException
caught MyException

而是输出：

throwing AnotherException
caught AnotherException
throwing MyException
caught Exception

有人可以解释为什么它“跳过”catch(MyException $e)？

谢谢.