display unique values from MYSQL PHP

I am trying to get the isbn number of a book from a database, but with my command it is giving me an associative array...How do I get it to return just the isbn number in the array.
Here is my code

function view_all_names($db)

{

$query = "SELECT  isbn from books";



$statement = $db->prepare($query);

$statement->execute();

$result = $statement->fetchAll();

$statement->closeCursor();

return $result;      



}