Get which checkboxes have been checked in PHP

I would like to get all checked checkboxes in a form, so if there were a list of checkboxes with the values 'a', 'b', 'c' and 'd' and only checkbox 'a' and 'c' were checked, my query would only select 'a' and 'c' from the desired table.

What would be the simplest way of accomplishing this prefferably in PHP?