Check if file name in current URL is not equal to $a or $b

I am attempting to check if the file name of the current URL is not equal to two values:

$fn = basename($_SERVER['PHP_SELF']);

if ($fn !== 'blah.php' OR $fn !== 'bleh.php') {
    // do something
}

For some reason, this if statement fails.

However, if I use simply:

$fn = $_SERVER['PHP_SELF'];

if ($fn !== 'blah.php') {
   // do something
}

This works fine.

Any ideas what I am doing wrong?

Thank you for your help in advance!