isset() Assumptions

register_setting() can be really handy, but take note! The first time your option gets saved, it’ll pass through the santize callback twice. With most input, it won’t matter; and if you’re explicit, it won’t matter either. But under the right conditions, it will.

Suppose your register_setting() looks like this:

register_setting( $option_group, 'checkbox', 'sanitize' );

And you have a pair of checkboxes like this:

<input type="checkbox" name="checkbox[one]" value="1" />
<input type="checkbox" name="checkbox[two]" value="1" />

And finally, your sanitize callback looks like this

function sanitize( $input ) {
	$input['one'] = isset( $input[ 'one' ] ) ? true : false;
	$input['two'] = isset( $input[ 'two' ] ) ? true : false;
	return $input;
}

See the problem?

On the very first save, both $input['one'] and $input['two'] will be true, no matter what. Say checkbox one is unchecked, when it fist passes through, $input['one'] is set to false – as expected. But when the input is passed through sanitize the second time, $input['one'] is set (to false) and thus the $input['one'] is changed to true.

So the moral of the story is: be more explicit, don’t make assumptions.