Drupal 6 template.php theme_search_theme_form(), theme_search_block_form()

template.php

<?php
/**
 * Implements theme_search_theme_form()
 */
function p6_search_theme_form($form) {
  $form['search_theme_form']['#title'] = t('Search this site');
  $form['search_theme_form']['#default_value'] = '';
  $form['search_theme_form']['#attributes']['title'] = t('Enter the terms you wish to search for.');
  $form['search_theme_form']['#description'] = t('Enter the terms you wish to search for.');

  $form['submit']['#value'] = t('Search');

  return drupal_render($form);
}

/**
 * Implements theme_search_block_form()
 */
function p6_search_block_form($form) {
  $form['search_block_form']['#title'] = t('Search this site');
  $form['search_block_form']['#default_value'] = '';
  $form['search_block_form']['#attributes']['title'] = t('Enter the terms you wish to search for.');
  $form['search_block_form']['#description'] = t('Enter the terms you wish to search for.');

  $form['submit']['#value'] = t('Search');

  return drupal_render($form);
}