From International PHP Magazine Issue: 3.2004 - by Robert Peake

"This article, PHP Cryptography - An Introduction Using Mcrypt, will define two-way key cryptography and explain how it differs from other well known PHP functions, like md5 and rot13, and when it is appropriate to use one-way hashing or two-way encryption. Then we will step through installing mcrypt as a dynamically loadable extension. We will explore two applications: encrypting cookies, and encrypting database information (such as credit card numbers). The article will point out some of the security implications of creating an encryption/decryption scheme in a plain text scripting language, and offer solutions such as encoding, source-compiling commands, and authoring new extensions."