HELP: Could not use File Helper to remove file

[eluser]joneslee[/eluser]
Hi everyone,
I am very new to PHP and CI so please be gentle if my question might sound clumsy. I couldn't remove a file on my server located at /sample/test.txt (owned by apache:apache and octal is 764). Below is my code:

Code:

$file = './sample/test.txt';

delete_files($file);

Just wondering if I am missing any important piece of the formula...