How can I make a condition opposite in MySQL?

I have this query:

$db
->prepare("UPDATE users 
            SET reputation = reputation + 
                             CASE
                               WHEN id = ? THEN 2
                               WHEN id = ? AND ? THEN 15
                             END
            WHERE id IN (?, ?); ")
->execute(array($author_ques_id, $author_ans_id, $bool, $author_ans_id, $author_ques_id));

Please focus on this condition:

WHEN id = ? AND ? THEN 15
//              ^ this is containing a boolean value

And I want to opposite the value of that condition. I mean I want to run that condition when $bool is false and don't run it when it is true.

I can do that before passing by php:

$bool = !$bool;

But I want to know can I do that in the query by MySQL?