interesting I think I'd like to know this to

Very good question

I've looked around the code, but cannot find the reference. 

Any updates on above post???????

this sounds familiar ;-) i think i coded this interface for some client many many months ago....

There is logout hook "trigger_elgg_event('logout','user',$_SESSION['user'])", I think one can register to this hook, an destroy the session by his/her own hook and then redirect to another page.

(original)

$result = logout();
if ($result) {
    system_message(elgg_echo('logoutok'));
} else {
    register_error(elgg_echo('logouterror'));
}

(modified)

$result = logout();
if ($result) {
    forward (url);
} else {
    register_error(elgg_echo('logouterror'));
}

any suggestions on this how can i redirect to particular url after logout by upper code.

@David Moon, TwizaNex Logout Page 1.8 for Elgg 1.8 has been released. I hope this can help you to redirect your users to another page at logout. For more information, get the plugin here  Hope it helps.

Thanks.