Drupal - Search Button + Font Awesome

template.php

<?php

/**
 * Implements hook_form_alter
 * @param $form
 * @param $form_state
 * @param $form_id
 */

function hook_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'search_block_form') {
    $form['search_block_form']['#attributes']['placeholder'] = t('Search');

    $form['button'] = array(
      '#type' => 'item',
      '#markup' => '<button type="submit" id="edit-submit-btn" name="op" class="form-submit"><i class="fa fa-search"></i></button>',
      '#weight' => 1000,
    );

  }
}

/**
 * Implements hook_preprocess_html
 * @param $variables
 */
function hook_preprocess_html(&$variables) {
  drupal_add_css('http://netdna.bootstrapcdn.com/font-awesome/4.0.3/css/font-awesome.css', array('type' => 'external'));
}