Mocking public method of testing object

I have a class that has two public method. It looks something like following:

class IntRequest
{
    public function updateStatus()
    {
        $isValid = $this->checkValidity();

        // ... next is a complex logic that use $isValid
    }

    /**
     * @return bool
     */
    public function isValid()
    {
        // another complex logic
    }
}

I need to test a first function - IntRequesr::updateStatus; however I need to run to tests. The first one with IntRequests::isValid returns false and the second one with true as a result of IntRequests::isValid

I try to mock that function but tests run with calling actual IntRequests::isValid not mocked one.

My testing code is

$intRequest = new IntRequests;

$mock = m::mock($intRequest);
$mock->shouldReceive('isValid')
    ->once()
    ->andReturn(true);

$res = $mock->updateStatus();

$this->assertTrue($res);

I've try to call $res = $intRequest->updateStatus() instead of $res = $mock->updateStatus() but with no luck.

So, I am wondering is it possible to mock function that is called inside testing method?