How can I build a redirect from submitted form values?
Okay I have figured it out by myself.

Here is what I needed: 

function form_findYourCity_submit($form, &amp;$form_state) {

  $city = $form_state['values']['city_name'];

  $form_state['redirect'] = [
    'showCity',
    [
      'query' =&gt; [
        'city' =&gt; $city,
      ],
    ],
  ];

}