How to use Unlink() function

I'm trying to use PHP unlink() function to delete away the specific document in the folder. That particular folder has already been assigned to full rights to the IIS user.

Code:

$Path = './doc/stuffs/sample.docx';
if (unlink($Path)) {    
    echo "success";
} else {
    echo "fail";    
}

It keep return fail. The sample.docx does reside on that particular path. Kindly advise.