image digit to check password

Hi, There is a trend on internet that when you want to change your password, you need to type : 1. the former password. 2. the new one 3. a 2nd time the new one to confirm that no mistake has been done on step 2. however, several website also propose an additional security to avoid hacking/brute force, they display a 4 digit image with a "random" string including number and letters... something like A3P23. if user do not respect case and string, password is not changed. I would like to know how such system works and if a tutorial exist about that ? thanks a lot, -- Alain ------------------------------------ Windows XP SP2 PostgreSQL 8.1.4 Apache 2.0.58 PHP 5