How do I use pdo's prepared statement for order by and limit clauses?

I want to use a prepared statement in which the passed-in parameters are for the ORDER BY and LIMIT clauses, like so:

$sql = 'SELECT * FROM table ORDER BY :sort :dir LIMIT :start, :results';
$stmt = $dbh->prepare($sql);
$stmt->execute(array(
     'sort'  => $_GET['sort'], 
     'dir'  => $_GET['dir'], 
     'start'  => $_GET['start'],
     'results' => $_GET['results'],
     )
    );

But $stmt->fetchAll(PDO::FETCH_ASSOC); returns nothing.

Can someone point out what's the wrong thing I am doing? Can it be done? If not,what should I reference for a complete list of clauses where parameters can be used?