I have a checkbox array and would like to have the submit button check to make sure that at least 1 of the boxes is checked.

Here's my example checkbox code for use with php:
<input type="checkbox" name="field[certificate]" value="$applicant[apid]">
<input type="checkbox" name="field[interview]" value="$applicant[apid]">
How would I go about creating such a thing?