How test methods that are not of my test class with PHPUnit, mock, stub

I have this class

<?php 

class Password 
{
    protected function checkPassword()
    {
        $this->callExit();
    }

    protected function callExit()
    {
    exit;
    }    
}

and this is my test:

 public function testAuthorizeExitsWhenPasswordNotSet()
    {


        $badCode = $this->getMockBuilder(Password::class)
            ->setMethods(array('callExit'))
            ->getMock();

        $badCode->expects($this->once())
            ->method('callExit');

        $badCode->checkPassword();

    }

In the earlier class the callExit method is of Password class. My question is, Can I test methods that aren't of the Password class ?

For example in checkPassword method:

protected function checkPassword()
{
    $user = new User;

    $this->callExit();

    $user->fillOut();

}

I want to do a mock for fillOut method, How do I it?

Help me plis !!