CodeIgniter - Why does encrypting with the same key produce different results?

Question

I use codeigniter a lot, however I am not really understanding why when I use the encryption library in version 3 the encryption string never comes out the same, even using the

Answer 1

Codigniter documentation:

DO NOT use this or any other encryption library for user password storage! Passwords must be hashed instead, and you should do that via PHP’s own Password Hashing extension.

http://www.codeigniter.com/userguide3/libraries/encryption.html

Fully explained here:

http://php.net/manual/en/faq.passwords.php