[Codeigniter][Solved] In order to use the Session class you are required to set an encryption key in your config file.

Error :
"In order to use the Session class you are required to set an encryption key in your config file."
As the sentence said, this error caused by missing the encryption key.
You need go to edit document config.php in this path: ci/application/config/,

Change the text from

$config['encryption_key'] = "";

To this:

$config['encryption_key'] = "YOUR KEY";

(< ---- Any word is okay in the Quotation marks, you can use "YOUR KEY", "HAHAHA", "Hello World" etc)

Reference link:
http://stackoverflow.com/questions/5385524/in-order-to-use-the-session-class-you-are-required-to-set-an-encryption-key-in-y
http://ellislab.com/codeigniter/user-guide/libraries/encryption.html