PHP encrypt and decrypt

http://naveensnayak.wordpress.com/2013/03/12/simple-php-encrypt-and-decrypt/
http://stackoverflow.com/questions/5422757/letter-shift-with-php
http://www.staroneweb.co.in/simple-effective-encrypt-decrypt-with-php/
http://blog.justin.kelly.org.au/simple-mcrypt-encrypt-decrypt-functions-for-p/
http://www.westhost.com/contest/php/function/simple-encrypt-decrypt-using-a-key/227