PHP form is sending robot type responses on my form.

Another trick is to ask a question that only a human can answer like "What is the color of red wine?"  Or "Who is burried in Grant's Tomb?"

Program your PHP script to reject incorrect answers and abort form processing. This works for robot submissions but unfortunately there's nothing much you can do about human spammers.

Do you check for the correct answer on the server?

Maybe choose random numbers to give a different answer each time

I use a hidden field that bots tend to populate. On the server, I reject ny posts with a populated hidden field.

yes it looks like  a bot... so to protect from it, you will have to use a human action needed.

captcha is one of them, https://en.wikipedia.org/wiki/CAPTCHA  but today most of the bot knows how to workaround it...

you can also place a honey pot for bots https://www.thryv.com/blog/honeypot-technique/

You can also work on a technique that consists in asking a question related to a picture that proposes a mathematical operation to solve, or any other type of question that links a visual and a logic.
In any case, there is no perfect and infallible solution.

Your test is easy enough to circumvent, the same logic in the form of an image is already more complex to circumvent.

I's probably not a hack, just a more sophisticated spam bot encountering a relatively lackluster "human test".

It's pretty unlikely that there's anything to worry about.