MySQL and PHP - insert NULL rather than empty string

I have a MySQL statement that inserts some variables into the database. I recently added 2 fields which are optional ($intLat, $intLng). Right now, if these values are not entered I pass along an empty string as a value. How do I pass an explicit NULL value to MySQL (if empty)?

$query = "INSERT INTO data (notes, id, filesUploaded, lat, lng, intLat, intLng)
          VALUES ('$notes', '$id', TRIM('$imageUploaded'), '$lat', '$long', 
                  '$intLat', '$intLng')";
mysql_query($query);