how do I get all checkbox variables even if not checked from HTML to PHP?

I noticed that PHP seems to return only values of checked checkboxes. I would like to see a list of checkboxes, not just values of checked checkboxes. Is there a way to detect variables of unchecked boxes?

I asked because I want to be able to update settings. For example, I have a few options that are already checked but if an user decides to uncheck an option, I need to know that unchecked value so I can update the option to be disabled.