更改数据库中复选框的值

I have a form with check boxes in it. I didn't set the value of the check boxes I always thought the value was either 'on' or '' (empty). I have the value 'on' passing to the database. The problem is when I unchecked the box it isn't changing the value in the database to '' (empty). The rest of the values of the form, such as input boxes are being changed. Here is my code on my check boxesI have a form with check boxes in it. I didn't