// Add some cool text to the search block form
function acq_margintest_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'search_block_form') {
    // HTML5 placeholder attribute
    $form['search_block_form']['#attributes']['placeholder'] = t('enter search terms');
  }
}

html5 placeholder drupal search form

drupal

PHP