It’s not that I’m speaking about.
Have a look at this:
http://www.phpnuke.org/modules.php?name=News&file=article&sid=3974
and for a better explanation:
http://www.phpnuke.org/modules.php?name=News&file=article&sid=6769
And search for “short URL” at PHPnuke.
I’m speaking of the links you see on your browser and not about html allowed on the site.