What are some practices I should use in a product registration system I'm building? I likely can't stop all malicious hacking, but I'd like to slow them down a great deal. (Note, I know only PHP.) I'm talking about things like encrypting traffic, testing the encryption from hacking like a man-in-the-middle attack, etc. The other concern I have is that this needs to work on most PHP5-based web hosting environments, which may not have mcrypt installed.