Does folder exist

I want to create a simple search if a "folder" exists. If it doesn't I want a notification similar to form_validation error.

I've developed the coding below, the 1st line works but the 2nd line doesn't work.

PHP Code:

if(!is_dir("members/$boss"))
$this->form_error("$boss", 'The boss username does not exist'); 


I want an error notification output - The boss username does not exist'

But if the folder does exist then the process continues.

I've searched many sites however some come close but not precisely what I want.

Can anybody guide me?