Flags are not used with FILTER_CALLBACK.

'id' => [
  'filter' => FILTER_CALLBACK,
  'options' => function($v){ return is_scalar($v) ? $v : null; },
]

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.

···
        var_dump(
            filter_input_array(
            INPUT_GET,
            [
                'id' => [
                    'filter' => FILTER_CALLBACK,
                    'options' => function($v){ return is_scalar($v) ? $v : null; }
                    ]
            ]
            )
        );
        exit();

/?id=2
array(1) {
  ["id"]=>
  string(1) "2"
}



/?id[]=3&id[]=4
array(1) {
  ["id"]=>
  array(2) {
    [0]=>
    string(1) "3"
    [1]=>
    string(1) "4"
  }
}
···
I must limit ID must be scalar, not array, filter cannot control!

Not just the Documentation Problem problem, but Bug still has problems.

Perhaps classifying this as a doc bug was premature. While the flags are deliberately not used, I see no way to use FILTER_CALLBACK and enforce a scalar or array: the callback is called on the non-array (leaf) values, so even with
  ?id[0][]=3&id[1][]=4
the callback only receives the values 3 and 4.

The obvious workaround is to apply two filters: one with filter_input_array and FILTER_REQUIRE_SCALAR, then another with filter_var_array and FILTER_CALLBACK.

i'm feel no happy.