Access JSON object returned from server-side processing

Hi,

I would like to access JSON object returned by DataTable from PHP script. Right now I can add some data to the JSON (in PHP) like this

$output = array(
"table_header"=>$header, 
"sEcho" => intval($_GET['sEcho']),
"iTotalRecords" => 0,
"iTotalDisplayRecords" => 0,
"aaData" => array()
);

echo json_encode( $output );

and DataTables process it just fine (despite the additional item), but I can't find a way to read the returned object in javascript. Any idea how should I proceed to read the value of 'table_header' from the above code sample? Thank you.