search_form_alter NOT BLOCK
The hook you want is hook_form_FORM_ID_alter():

/**
 * Implements hook_form_FORM_ID_alter().
 */
function MYMODULE_form_search_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  // Code here
}


Alternatively you could use hook_form_alter():

/**
 * Implements hook_form_alter().
 */
function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'search_form') {
    // Code here
  }
}