Spent the morning banging my head against my desk due to Drupal 7’s Forms API. I was trying to convert a regular POST-form to GET without any success, until I found this useful article http://eworldproblems.mbaynton.com/2015/03/handling-get-method-submissions-with-drupal-7s-forms-api/

Drupal 7 Forms API reference: https://api.drupal.org/api/drupal/developer%21topics%21forms_api_reference.html/7.x