Get number of posts in a topic PHP

Programmer Question

How do I get to display the number of posts on a topic like a forum. I used this... (how very noobish):

function numberofposts($n)
{
    $sql = "SELECT * FROM posts
            WHERE topic_id = '" . $n . "'";

    $result = mysql_query($sql) or die(mysql_error());          
    $count = mysql_num_rows($result);           

    echo number_format($count);
}

The while loop of listing topics:

Although it is a bad method of doing this... All I need is to know what would be the best method, don't just point me out to a website, do it here, because I'm trying to learn much. Okay? :D

Thanks.

Find the answer here