First of all, this is HTML, not PHP: <a href=utilisateur_acceuil_"$util".php?></a>;. That will cause a parse error if you put it in the middle of php code.
The error you're getting is because headers have to be sent before any other output. I'm assuming you're echoing (or otherwise sending to the browser) the <HTML><HEAD> etc. tags. That's why you're getting the error about headers.
As far as a solution, I'm not real sure exactly what you're trying to do. It almost looks like maybe what you need to do is include() the page you're trying to forward to. More info about include: http://www.php.net/include .