Description:
------------
I think, it would be useful to add new argument $invert to array_filter() function. It will be most useful to use built-in functions as a callback. For example this call:

    $result = array_filter($values, 'is_null', true);

should to _remove_ all NULLs from array.

May be the parameter should be named $positive and have default value TRUE (and FALSE for inversion) to make code readable.

Test script:
---------------
$values = array(42, 'foo', false, null, array(), '');
var_dump(
  array_filter($values, null, true), // no callback - leave false value
  array_filter($values, 'is_null', true) // leave non-null values
);

Expected result:
----------------
array(5) {
  [0]=>
  bool(false)
  [1]=>
  NULL
  [2]=>
  array(0) {
  }
  [3]=>
  string(0) ""
}
array(5) {
  [0]=>
  int(42)
  [1]=>
  string(3) "foo"
  [2]=>
  bool(false)
  [3]=>
  array(0) {
  }
  [4]=>
  string(0) ""
}