public static function instance()
 {
     if (null == self::$instance) {
         self::$instance = new self();
     }
     return self::$instance;
 }
<?php

/**
 * Plugin Name: Woo Force Coupon
 * Plugin URI: http://kowsarhossain.com/
 * Description: A lightweight WooCommerce based plugin to force the customer to enter a coupon code during checkout
 * Version: 1.0.0
 * Author: Md. Kowsar Hossain
 * Author URI: http://kowsarhossain.com
 * Text Domain: woo-force-coupon
 * License: GNU General Public License v3.0
 * License URI: http://www.gnu.org/licenses/gpl-3.0.html
 *
 */
if (!defined('WPINC')) {
    die;
}
// exit if WooCommerce not activated
if (!in_array('woocommerce/woocommerce.php', apply_filters('active_plugins', get_option('active_plugins')))) {
    return;
}
define('WFCFOA_BASENAME', plugin_basename(__FILE__));
if (is_admin()) {
    require_once dirname(__FILE__) . '/admin/class-settings-api.php';
    require_once dirname(__FILE__) . '/admin/class-woo-force-coupon-admin.php';
    FOA_Woo_Force_Coupon_Admin::instance();
}
require_once dirname(__FILE__) . '/public/class-woo-force-coupon.php';
FOA_Woo_Force_Coupon::instance();