Assuming you are using apache, you can change the default “tell everything” behavior using ServerTokens and ServerSignature :
ServerTokens Prod
ServerSignature Off
HTTP- . ServerTokens:
14:45:52 bartley:~ > curl -I http://www.test.com
HTTP/1.1 200 OK
Date: Mon, 16 May 2011 13:54:48 GMT
Server: Apache/2.2.15 (EL) DAV/2 PHP/5.2.16 mod_ssl/2.2.15 OpenSSL/0.9.8e-fips-rhel5 mod_perl/2.0.4 Perl/v5.8.8
Accept-Ranges: bytes
Content-Length: 16457
Cache-Control: max-age=300, must-revalidate
Expires: Mon, 16 May 2011 13:59:48 GMT
Vary: Accept-Encoding,Cookie
Connection: close
Content-Type: text/html; charset=UTF-8
.. Prod:
14:44:25 bartley:~ > curl -I http://www.test.com
HTTP/1.1 200 OK
Date: Mon, 16 May 2011 13:54:19 GMT
Server: Apache
Accept-Ranges: bytes
Content-Length: 16457
Cache-Control: max-age=300, must-revalidate
Expires: Mon, 16 May 2011 13:59:19 GMT
Vary: Accept-Encoding,Cookie
Connection: close
Content-Type: text/html; charset=UTF-8
EDIT: @Marc, HTTP-, PHP X-Powered-By. , expose_php = Off php.ini.