Simplest two-way encryption using PHP

Question

What is the simplest way of doing two way encryption in common PHP installs?

I need to be able to encrypt data with a string key, and use the same key to decrypt on

Answer 1

Use mcrypt_encrypt() and mcrypt_decrypt() with corresponding parameters. Really easy and straight forward, and you use a battle-tested encryption package.

EDIT

5 years and 4 months after this answer, the mcrypt extension is now in the process of deprecation and eventual removal from PHP.