You should add quotes to strings and use the [php] tag:
To keep secret that directory, I add that you should NEVER click on external links (like plugin author's site) from the Admin CP.
The link to your ACP is sent to that site via the referral header and is no longer secret.
As a plugin author, I can tell you!
If you want to visit plugin author's site, you should copy link URL and past it in a new tab.
PHP Code:
$config['admin_dir'] = 'iamsecured';
To keep secret that directory, I add that you should NEVER click on external links (like plugin author's site) from the Admin CP.
The link to your ACP is sent to that site via the referral header and is no longer secret.
As a plugin author, I can tell you!
If you want to visit plugin author's site, you should copy link URL and past it in a new tab.