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

/**
 * Plugin Name: Omise Payment Gateway
 * Plugin URI: https://www.omise.co/woocommerce-plugin
 * Description: Omise WooCommerce Gateway Plugin is a wordpress plugin designed specifically for WooCommerce. The plugin adds support for Omise Payment Gateway payment method to WooCommerce.
 * Version: 1.1.2
 * Author: Omise
 * Author URI: https://www.omise.co
 *
 * Copyright: Copyright 2014-2015. Omise Co., Ltd.
 * License: MIT
 * License URI: https://opensource.org/licenses/MIT
 */
defined("ABSPATH") or die("No direct script access allowed.");
define("OMISE_PROTOCOL_PREFIX", "https://");
define("OMISE_VAULT_HOST", "vault.omise.co");
define("OMISE_API_HOST", "api.omise.co");
define("OMISE_WOOCOMMERCE_PLUGIN_VERSION", "1.1.2");
require_once "omise-util.php";
require_once "omise-api-wrapper.php";
require_once "omise-wc-gateway.php";
require_once "omise-wc-myaccount.php";
require_once "omise-wp-admin.php";
add_action("init", "register_omise_wc_gateway_post_type");
add_action("plugins_loaded", "register_omise_wc_gateway_plugin", 0);
add_action("plugins_loaded", "prepare_omise_myaccount_panel", 0);
add_action("plugins_loaded", array(Omise_Admin::get_instance(), "register_admin_page_and_actions"));
<?php

defined('ABSPATH') or die("No direct script access allowed.");
/**
 * Plugin Name: Omise Gateway Wordpress plugin
 * Plugin URI: http://docs.omise.co/omise-wp
 * Description: Allows easy integrating the Omise Payment gateway
 * Version: 1.0.2
 * Author: Omise Team
 * Author URI: https://www.omise.co
 * License: Copyright 2014. Omise Co.,Ltd.
 */
define("OMISE_PROTOCOL_PREFIX", "https://");
define("OMISE_VAULT_HOST", "vault.omise.co");
define("OMISE_API_HOST", "api.omise.co");
define("OMISE_WOOCOMMERCE_PLUGIN_VERSION", "1.0.2");
require_once 'omise-util.php';
require_once 'omise-api-wrapper.php';
require_once 'omise-wc-gateway.php';
require_once 'omise-wc-myaccount.php';
require_once 'omise-wp-admin.php';
add_action('plugins_loaded', 'register_omise_wc_gateway_plugin', 0);
add_action('plugins_loaded', 'prepare_omise_myaccount_panel', 0);
add_action('plugins_loaded', array(Omise_Admin::get_instance(), 'register_admin_page_and_actions'));