How do I include ? and = in a form redirection?
In Drupal 6, instead of setting `$form_state['redirect'] with a string, you set it with an array with tree items.

$form_state['redirect'] = array('some-path/search', 'keys=' . check_plain($form_state['values']['input']), '');


The last item would be a value used as anchor. As in your case you don't need it, pass an empty string.