Simplest two-way encryption using PHP

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 the other end. The security isn't as big of a concern as the portability of the code, so I'd like to be able to keep things as simple as possible. Currently, I am using an RC4 implementation, but if I can find something natively supported I figure I can save a lot of unnecessary code.