checkboxes

I have a form where I am dynamically building a bunch of checkbox inputs. Then when submitted, I need to update the db. Problem is, of course, if the checkbox is unchecked then the $_POST doesn't have the value in the array. How can I get the value from $_POST even though it's unchecked? For example, here would be the generated form: <input type="checkbox" name="cars[2]" value="1">Car 1 <input type="checkbox" name="cars[3]" value="1">Car 2 <input type="checkbox" name="cars[4]" value="1">Car 3 <input type="checkbox" name="cars[5]" value="1">Car 4 Then in the update I loop through: foreach($_POST[cars] as $id=>$value){ //update vehicles set value to yes or no } But, if the unchecked values aren't in $_POST, how do I know to update them??? Thanks!