RWMB_Loader::init PHP Method

    public function init()
    {
        $this->constants();
        // Register autoload for classes
        require_once RWMB_INC_DIR . 'autoloader.php';
        $autoloader = new RWMB_Autoloader();
        $autoloader->add(RWMB_INC_DIR, 'RW_');
        $autoloader->add(RWMB_INC_DIR, 'RWMB_');
        $autoloader->add(RWMB_INC_DIR . 'fields', 'RWMB_', '_Field');
        $autoloader->add(RWMB_INC_DIR . 'walkers', 'RWMB_Walker_');
        $autoloader->register();
        // Plugin core
        new RWMB_Core();
        if (is_admin()) {
            // Validation module
            new RWMB_Validation();
            $sanitize = new RWMB_Sanitizer();
            $sanitize->init();
        }
        // Public functions
        require_once RWMB_INC_DIR . 'functions.php';
    }

<?php

/**
 * Plugin Name: Meta Box
 * Plugin URI: https://metabox.io
 * Description: Create custom meta boxes and custom fields for any post type in WordPress.
 * Version: 4.9.8
 * Author: Rilwis
 * Author URI: http://www.deluxeblogtips.com
 * License: GPL2+
 * Text Domain: meta-box
 * Domain Path: /languages/
 */
if (defined('ABSPATH') && !defined('RWMB_VER')) {
    require_once dirname(__FILE__) . '/inc/loader.php';
    $loader = new RWMB_Loader();
    $loader->init();
}