Unable to save variable(s) from a custom form
Modify your example_form_form1_submit function as follows

function example_form_form1_submit(&amp;$form, &amp;$form_state) {
  variable_set('id_event', $form_state['values']['id_event']);
  $id_event = variable_get('id_event', '');
  drupal_set_message($id_event);
}