Encrypted field

Hi PEARs, I'm using formbuilder and as showed in wiki example I must encrypt some data in database. But I'cant use md5 because data must be recoverable. Which type of encryption should I use? I can use MySQL functions, php functions or any external clases (RSA or something). And, how would you do this? Is the example at the wiki the unique way? I have a lot of entities and I it could ble clearer to use some kind of function, instead of copy-paste-modify. Thank U, Rodrigo.