foreach($files as $file) {
            if(is_file($file)) {
                unlink($file);
            }
        }
        delete_option('perfmatters_used_css_time');
    }

    //clear used css file for specific post or post type
    public static function clear_post_used_css() {
        if(empty($_POST['action']) || empty($_POST['nonce']) || empty($_POST['post_id'])) {
            return;
        }

        if($_POST['action'] != 'perfmatters_clear_post_used_css') {
            return;
        }

        if(!wp_verify_nonce($_POST['nonce'], 'perfmatters_clear_post_used_css')) {
            return;
        }

        $post_id = (int)$_POST['post_id'];
        $post_type = get_post_type($post_id);
        $path = $post_type == 'page' ? 'page-' . $post_id : $post_type;

        $file = PERFMATTERS_CACHE_DIR . 'css/' . $path . '.used.css';
        if(is_file($file)) {
            unlink($file);
        }

        wp_send_json_success();
        exit;
    }

    //add admin bar menu item
    public static function admin_bar_menu(WP_Admin_Bar $wp_admin_bar) {

        if(!current_user_can('manage_options') || !perfmatters_network_access()) {
            return;
        }

        $type = !is_admin() ? self::get_url_type() : '';

        $menu_item = array(
            'parent' => 'perfmatters',
            'id'     => 'perfmatters-clear-used-css',
            'title'  => __('Clear Used CSS', 'perfmatters') . ' (' . (!empty($type) ? __('Current', 'perfmatters') : __('All', 'perfmatters')) . ')',
            'href'   => add_query_arg(array(
                'action'           => 'perfmatters_clear_used_css',
                '_wp_http_referer' => rawurlencode($_SERVER['REQUEST_URI']),
                '_wpnonce'         => wp_create_nonce('perfmatters_clear_used_css'),
                'type'             => $type
            ), 
            admin_url('admin-post.php'))
        );

        $wp_admin_bar->add_menu($menu_item);
    }

    //display admin notices
    public static function admin_notices() {

        if(get_transient('perfmatters_used_css_cleared') === false) {
            return;
        }

        delete_transient('perfmatters_used_css_cleared');
        echo '<div class="notice notice-success is-dismissible"><p><strong>' . __('Used CSS cleared.', 'perfmatters' ) . '</strong></p></div>';
    }

    //clear used css from admin bar
    public static function admin_bar_clear_used_css() {

        if(!isset($_GET['_wpnonce']) || !wp_verify_nonce(sanitize_key($_GET['_wpnonce']), 'perfmatters_clear_used_css')) {
            wp_nonce_ays('');
        }

        if(!empty($_GET['type'])) {

            //clear specific type
            $file = PERFMATTERS_CACHE_DIR . 'css/' . $_GET['type'] . '.used.css';
            if(is_file($file)) {
                unlink($file);
            }
        }
        else {

            //clear all
            self::clear_used_css();
            if(is_admin()) {
                set_transient('perfmatters_used_css_cleared', 1);
            }
        }

        //go back to url where button was pressed
        wp_safe_redirect(esc_url_raw(wp_get_referer()));
        exit;
    }
}