Display current viewers web address in sidebar

I keep trying to display a website address of people who have previously commented on my site into my sidebar but with no luck so far. I’m trying to use this code but I don’t know what i am doing wrong.
<?php
$commenter_url = $wpdb->get_var("SELECT comment_author_url
FROM wp_comments
ORDER BY comment_author_url
LIMIT 0 , 30");

if(!empty($commenter_url)){
echo "<center><b><font color=#cc66cc>Your web url is, $commenter_url!</font></b></center>";
}else{
echo "<center><b><font color=#cc66cc>No web url available!</font></b></center>";
}
?>
Please be patient with me as I am still trying to learn. Thank you for any help you can give.