How to have checkbox items create a list

I am new to PHP and the project I am working on is a codeigniter php form writing back to a FileMaker database.

The checkboxes should be populating as a list field.

Here is my form

<input name="fieldname[]" value="value1"<?php echo (set_checkbox('fieldname', $join->fieldname))?> type="checkbox" class="form-check-input"> value1
<input name="fieldname[]" value="value2"<?php echo (set_checkbox('fieldname', $join->fieldname))?> type="checkbox" class="form-check-input"> value2

There are about 7 checkboxes.

I assume that I have to setup a foreach loop to get it to go thought to FileMaker's field, but unsure how to do that.

If I take away the array, the last one selected goes through to the layout.

Any help would be great.