how to check for null in string

[PJ]

I know this is not working, but how can I check for NULL; or how can I
configure my field "sub_title" so I can check if $booksub_title contains
anything or is empty.
This problem as been breaking my back...don't know what to put in as a
default to be able to check against string input (VARCHAR) or should I
use another type?

if ($booksub_title != NULL) {
        echo "<td width='400'><b>$booktitle[$i]</b><br>
$booksub_title[$i]</b>";
            }
        else {echo "<td width='400'><b>$booktitle[$i]</b>";
            }

-- 

Phil Jourdan --- pj@stripped
   http://www.ptahhotep.com
   http://www.chiccantine.com