Drupal 7 - Search block alter

template.php

/**
 * Implements hook_form_FORM_ID_alter().
 */
function THEMENAME_form_alter(&$form, &$form_state, $form_id) {

  switch ($form_id)  {

    // Search block form
    case 'search_block_form':

      // Change title for search and hide it
      $form['search_block_form']['#title'] = t('Search');
      $form['search_block_form']['#title_display'] = 'invisible';

      // Button
      $form['actions']['submit'] = array('#type' => 'image_button', '#src' => base_path() . path_to_theme() . '/images/search-button.png');

      // Add extra attributes to the text box
      $searchtext = 'Search intranet';
      $form['search_block_form']['#default_value'] = t($searchtext);
      $form['search_block_form']['#attributes']['onblur'] = "if (this.value == '') {this.value = '$searchtext';}";
      $form['search_block_form']['#attributes']['onfocus'] = "if (this.value == '$searchtext') {this.value = '';}";

      // Prevent user from searching the default text
      $form['#attributes']['onsubmit'] = "if(this.search_block_form.value=='$searchtext'){ alert('Please enter a search term'); return false; }";
      break;

  }

}