You can send new HTTP headers using the header() function:
http://www.php.net/header
You can suppress the X-Powered-By: PHP/<version> header in PHP.ini. The only other header sent by PHP is the Content-Type header, and though you can change the default mime-type to whatever you want, I don't think you should. It's set to "text/html" by default.
Take care,
Nik
http://www.bigaction.org/