Sanitize Custom Field Checkbox

I ran into the same problem as described in this issue. Unfortunately that issue has been marked as resolved and closed, although I don’t think it is: Overwriting a backend PHP bug by frontend JavaScript is not a solution to me.

Here’s the problem: I have created my own custom field consisting of a checkbox and a text input. Once you have checked the the checkbox and saved the post, it will always be displayed as checked.

If I uncheck it, then save the post, the data are stored correctly ($value[‘checkbox’] is unset), but the checkbox is still displayed as checked. Saving the post again, of course $value[‘checkbox’]=’on’.