Ajax callback triggered by checkbox.
function ajax_forms_test_simple_form_checkbox_callback($form, $form_state) {
$commands = array();
$commands[] = ajax_command_html('#ajax_checkbox_value', (int) $form_state['values']['checkbox']);
$commands[] = ajax_command_data('#ajax_checkbox_value', 'form_state_value_select', (int) $form_state['values']['checkbox']);
return array(
'#type' => 'ajax',
'#commands' => $commands,
);
}