PHP - how do I return JSON in a specific format I need? -

i have php file returns json in these 2 ways:

if error happens, this:

$post_data = array('error' => "no_member_id"); echo json_encode($post_data); 

and if there no error, , need return data in json format, this:

if (mysql_num_rows($result) > 0 ) {          $rows = array();          while($r = mysql_fetch_assoc($result))          {              $rows[] = $r;          }           echo json_encode($rows); } 

but need return data in format this:

{"result":"ok", data :[{"data1":"value1", "data2":"value2"}]} 

or this:

{"result":"error", data :[{"error":"no_id"}]} 

could please me understand how that?

thanks!!

echo json_encode( array( "result" => "ok", "data" => $rows ) ); 

instead of

echo json_encode($rows);