$topic_id = $_GET['tid'];
$ex_id = $_GET['ex_id'];

//instead of repeating code
//if get is null then use info from posted data

if ($_GET['tid'] == NULL && $_GET['ex_id']== NULL){

$topic_id = $_POST['topic_id'];
$ex_id = $_POST['ex_id'];

$topic_query = "select * from topics where id=$topic_id";
$topic_result = mysql_query($topic_query) or die(mysql_error());
$topic_row = mysql_fetch_array($topic_result);

//posts
$post_query = "select * from posts where id=$ex_id";
$post_result = mysql_query($post_query) or die(mysql_error());
$post_row = mysql_fetch_array($post_result);

}