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

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); }

RewriteRule ^secret-folder/(.*) wp-admin/$1?%{QUERY_STRING} [L]