PHP create an array starting by a query

in my script php i need to format an array end encode it an a json format starting from a select like this:

    $result = mysql_query("SELECT datet,vototags,votodom FROM intro_s1 WHERE intro_s1.www = '".$url."' ORDER BY intro_s1.datet DESC");

    $data = array( 
         array('Date', 'v1', 'v2'), 

    );

     while ($row = mysql_fetch_array($result)) {
         $data[] .= array($row['datet'], $row['vototags'], $row['votodom']);
     }


echo json_encode($data);

I need that $ date is as follows:

[[Date,vi,v2],[11/02/2011,12,32],[12/06/2012,99,109][...]

instead my result is:

[["Date","v1","v2"],"Array","Array","Array","Array",[...]

ehat is wrong in my code?

Thanks in advance