function philosophy_search_form($form)
{
$home_dir = home_url('/');
$label = __('Search for:', 'philosophy');
$btn_label = __('Search', 'philosophy');
$search_form = <<<FORM
<form role="search" method="get" class="header__search-form" action="{$home_dir}">
<label>
<span class="hide-content">{$label}</span>
<input type="search" class="search-field" placeholder="Type Keywords" value="" name="s" title="{$label}" autocomplete="off">
</label>
<input type="hidden" name="post_type" value="book">
<input type="submit" class="search-submit" value="{$btn_label}">
</form>
FORM;
return $search_form;
}
add_filter('get_search_form', 'philosophy_search_form');