WordPress function to change admin URL from /wp-admin/* to /admin/*

admin-dir-rename.php

<?php
/**
 * Change Admin URL
 *
 * Copyright (C) 2010  hakre <http://hakre.wordpress.com/>
 *
 * USAGE:
 *
 * Copy the file into  wp-content/mu-plugins  directory and add the
 * following RewriteRule to your apache configuration or .htaccess:
 *
 * Apache: RewriteRule ^admin/(.*)$ wp-admin/$1 [QSA,L]
 * Nginx: rewrite ^/admin/(.*)? /wp-admin/$1 last;
 */

return ChangeAdminUrlPlugin::bootstrap();

class ChangeAdminUrlPlugin {
	private $renameFrom = 'wp-admin';
	private $renameTo = 'admin';
	static $instance;

	static public function bootstrap()
	{
		null === self::$instance
		&& self::$instance = new self();

		return self::$instance;
	}

	private function setCookiePath()
	{
		defined( 'SITECOOKIEPATH' ) || define( 'SITECOOKIEPATH', preg_replace( '|https?://[^/]+|i', '', get_option( 'siteurl' ) . '/' ) );
		defined( 'ADMIN_COOKIE_PATH' ) || define( 'ADMIN_COOKIE_PATH', SITECOOKIEPATH . $this->renameTo );
	}

	public function __construct()
	{
		$this->setCookiePath();
		add_action( 'init', [ $this, 'init' ] );
	}

	public function init()
	{
		add_filter( 'admin_url', [ $this, 'admin_url' ], 10, 3 );
		add_filter( 'network_admin_url', [
			$this,
			'network_admin_url'
		], 10, 3 );
	}

	public function admin_url( $url, $path, $blog_id )
	{
		$renameFrom = $this->renameFrom;
		$renameTo   = $this->renameTo;
		$scheme     = 'admin';
		$find       = get_site_url( $blog_id, $renameFrom . '/', $scheme );
		$replace    = get_site_url( $blog_id, $renameTo . '/', $scheme );
		( 0 === strpos( $url, $find ) )
		&& $url = $replace . substr( $url, strlen( $find ) );

		return $url;
	}

	public function network_admin_url( $url, $path )
	{
		$renameFrom = $this->renameFrom;
		$renameTo   = $this->renameTo;
		$scheme     = 'admin';
		$find       = network_site_url( $renameFrom . '/', $scheme );
		$replace    = network_site_url( $renameTo . '/', $scheme );
		( 0 === strpos( $url, $find ) )
		&& $url = $replace . substr( $url, strlen( $find ) );

		return $url;
	}
}

Thank you! This worked wonderfully! This was the only solution I found online which worked correctly! thanks again!

Seems it is not working properly
on open /admin/ it redirects on wp-login.php page
and /wp-admin/ is still available if type directy.