Drupal 7 Search Block Form Alter

gistfile1.php

<?php 

// Form Alter
function qhr_form_search_block_form_alter(&$form, &$form_state, $form_id) {
  $form['search_block_form']['#default_value'] = t('search'); // Set a default value for the textfield
  $form['actions']['submit']['#value'] = t('GO!'); // Change the text on the submit button
}