Form is not calling custom submit function
Try next:

function myModule_simple_form($form, &amp;$form_state){
  $form['firstname'] = array(
    '#title' =&gt; t('First name'),
    '#type'  =&gt; 'textfield',
  );
  $form['lastname'] = array(
    '#title' =&gt; t('Last name'),
    '#type'  =&gt; 'textfield',
  );

  $form['submit'] = array(
    '#value' =&gt; t('Search'),
    '#type'  =&gt; 'submit',
  );
  return $form;
}
function myModule_simple_form_submit($form, &amp;$form_state) {
  drupal_set_message($form_state['values']['firstname']);
}


Any rendered drupal form already have a submit function. Also try: drupal_set_message($form); in your hook_from_alter when $form_id = 'myModule_simple_form' too see where is default submit function. You can also add a custom submit funtion in array but it is necesary only if you need more than 1 submit function for your form. In all othre cases use just form_id_submit.