Why can't I use "%s" format value in the Wordpress function checked?

Question

I try to safe a checkbox with the Wordpress settings API. For that I use the checked() function. First I make my field with add_settings_field() and pass a view parameters in an array to the callback function.

In the callback function of my field I make an output with

printf(
        '<input type="checkbox" id="%2$s" name="%1$s[%2$s]" '. checked("on", '%3$s').'
         <label for="%2$s">%4$s</label>
         <hr /> ',
        $args['option_name'],
        $args['name'],
        $args['value'],
        $args['description']    
    );

Now normally the checkbox should be saved. BUT the check-function doesn't work.

When I print the parameter value %3$s then the value is "on" if the field is checked and nothing if the field is not checked. But the check-function never triggers even if %3$s is "on".

Only if I insert the "on" string checked("on", 'on') directly it returns checked.

So my question is: Why I can't use %3$s in the checked function?

Answer 1

Because the order of operations means you're passing that variable into your checked function before the printf function is even being run, which is what's parsing the enumerated variables.

Here's a possible solution haven't tested it though:

printf(
    '<input type="checkbox" id="%2$s" name="%1$s[%2$s]" %3$s)'.
    '<label for="%2$s">%4$s</label>' .
    '<hr />',
    $args['option_name'],
    $args['name'],
    checked("on", $args['value'], false ),
    $args['description']    
);