Yes it is, however, it will
- Make using a database far less worthwhile, as indexing, searching, collation etc, won't work any more (You can't meaningfully use WHERE, ORDER BY or joins on a column which contains encrypted data)
- Obfuscate your application massively
- Will offer limited security, as the key will still need to be stored somewhere on your web app server.
There are very specific cases where using some encryption (e.g. asymmetric encryption where the key required to decrypt the data is NOT held locally) or hashing (e.g. for password) on a limited subset of your data MAY be desirable, but general application-level encryption of all database data is going to make the database itself pretty redundant.
If you want to encrypt the data on your database server's discs, I recommend using an encrypted filesystem or block device and storing it on there.
If you want to encrypt data to/from the database, I recommend either using an SSL connection to the database or a VPN.
If you want to encrypt data to/from the client, I recommend using HTTPS and S/MIME or PGP for email.
Mark
