In Ubuntu open /etc/php5/apache2/php.ini with your favorite editor. Find the line that says expose_php = On and change it to say expose_php = Off. Save php.ini then restart Apache by running sudo service apache2 restart.
Simple isn't it? Now the bad guys can't see what version of PHP you are running, which will make breaking into your web server a little tougher.
What other things do you do to help protect your LAMP servers? Let me know in the comments.