Some time ago I wrote document describing common problem with cleaning up 
HTML. PHP manual states some little warning about topic but no solution on 
http://www.php.net/strip_tags 

Many websites are still vulnerable and similar problems happen not depending 
on programming language too often:

http://www.jes.ee/~tonu/strip.php 

Please do not start flamewar, I know some people might find this information 
useful. Yes, it is not "advanced" and this is why people often do not notice 
the problem.

  T=F5nu