PHP programming > Security

If your website has a “contact us” or “Sign me up to your newsletter” form it is possible that a spammer could use it to send spam from your server. This takes up your bandwidth, leaves your ISP with a mess to sort out (and for which they might charge you) and could in severe cases end up with emails from your domain being blocked from sending email.

This tutorial describes the process in creating a simple captcha image.

Its a formula - a way to take a message of an arbitrary length, and create a 128-bit "fingerprint" or "message digest" of the message. MD5 is a way to verify data integrity. On these forums, it comes up fairly often in discussions about storing user passwords and other sensitive data.