On the NETTUTS.com site there's a new tutorial posted looking at creating a "crypter" class in PHP - a handy class to make encryption and decryption of data simpler.
Think about what we might need a class like this for? We want to encrypt important data with a password for security reasons. We also want, as already mentioned, to be able to decrypt that data when necessary. Why should you use symmetric algorithms? It's easy; when you're offering a password sent via email or something like that, you need the password to be sent in plaintext. The hash algorithms are not reversible. Once you have hashed a string you can't decipher the original text from the hash.
He lays out his basic class with three methods - the constructor that sets up the key and algorithm and the encrypt and decrypt functions. These use to mcrypt functions to handle the heavy lifting.