PHP sqlsrv_query dynamic order by clause

Question

(Note that this is for MSSQL, not MYSQL)

Does anyone know why this doesn't work?

   $query = "SELECT * FROM table WHERE SeriesID = ? ORDER BY ? ?";
   $result = $conn->getData($query, array($seriesID,$sortBy,$sortOrder));

I don't see any errors, but no results are returned.

When I don't have the ORDER BY portion, it works. It's the second and third question marks that kill it.

$conn->getData() is calling sqlsrv_query() function...

There is a similar question here which I answered with a workaround, but I'd like to find out why this doesn't work as I think it should.