Here is one method that I have used when I wanted to know which checkboxes were present on a page. (Way back when I did web work.)

For each "thing" you want in the list produce two fields in the form, one a checkbox and one a "hidden" field. Call the checkbox fields something like "cb_N" and the hidden fields something like "hd_N" where N is a number from 1 to N and you have N "things".

When the data is submited you will get a list of all "hd_*" fileds but the checkboxes are only submitted if they are checked off. So you loop over all of the "hd_N" values and see if the "cb_N" was submitted or not for that number "N".