I've been having problems validating my websites with the W3C validator. My site uses PHP sessions, and this caused some errors :
Look at : http://validator.w3.org/check?uri=www.mathieumaes.be
The problem is that I don't control the "&PHPSESSID=" part of the URL. PHP adds this automatically when you use sessions.
Even the webhost couldn't resolve this, due to lack of knowledge about this matters, and lack of interest in W3c ;-)
Anyway, after a long search I've finally come up with a solution :
Add this line in the PHP script before session_start() :
result => the url is changed to ?amp;PHPSESSID= ....
Look at : http://validator.w3.org/check?uri=www.mathieumaes.com
Maybe you could add this information on the W3C website in the part "Ampersands in URLs" ?
Greeting to you all from Belgium