change admin directory

1. Add constant to wp-confing.php

define('WP_ADMIN_DIR', 'secret-folder');
define( 'ADMIN_COOKIE_PATH', SITECOOKIEPATH . WP_ADMIN_DIR);

2. Add below filter to functions.php

add_filter('site_url',  'wpadmin_filter', 10, 3);
 function wpadmin_filter( $url, $path, $orig_scheme ) {
  $old  = array( "/(wp-admin)/");
  $admin_dir = WP_ADMIN_DIR;
  $new  = array($admin_dir);
  return preg_replace( $old, $new, $url, 1);
 }

3. Add below line to .htaccess file
RewriteRule ^secret-folder/(.*) wp-admin/$1?%{QUERY_STRING} [L]

Done...!!!

Now your admin URL will be like: http://www.yourdomain.com/secret-folder/