OT: wp_get_referer / site_url for single.php

This is pretty offtopic on what i expect support for but if anyone could help i would be very grateful.

Currently i show my blog entries inside a lightbox without menu or sidebar. “Share this entry” is enabled so when someone shares something they get to the blog entry without any menu or sidebar or option to navigate.

I figured (from the wordpress docu) that i can use

$referrer = wp_get_referer();
if( strpos( $referrer, site_url() ) ){
// Was referred by current site.
} else {
// Was an external referrer
}
somehow, i guess in the single.php page…

As you probably figured: If the link is not refered to from the site itself it should show atleast a menu so the user can get back to the site.

Thanks in advance!