In this project I was using Drupal with Panels and Apache Solr search module. Here is a snippet that will add a font-awesome icon to the search button, in your apache solr panel search block.

function YOURTHEME_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'apachesolr_panels_search_block') {
    $form['actions']['submit']['#value'] = '< i class="fa fa-search">< /i > < span class="sr-only">Search< /span >';

In this case I am using Bootstrap with FontAwesome icons, so the classes “sr-only” will hide the text (except for screen readers) and the <i> tag with “fa fa-search” will add the search icon.