CodeIgniter is a popular web application framework for PHP that simplifies the process of building dynamic web sites and applications. It provides a variety of libraries and tools to help you manage sessions in your application.

To use sessions in CodeIgniter, you first need to load the session library. This can be done in your controller or in the autoload.php configuration file.

Once the session library is loaded, you can store and retrieve session data using the following methods:

• $this->session->set_userdata('key', 'value') - sets a session variable called ‘key’ with a value of ‘value’
• $this->session->userdata('key') - retrieves the value of the session variable ‘key’
• $this->session->unset_userdata('key') - removes the session variable ‘key’

For example, to set a session variable called ‘username’ with a value of ‘john’, you would use the following code:

$this->session->set_userdata('username', 'john');

To retrieve the value of the ‘username’ session variable, you would use the following code:

$username = $this->session->userdata('username');

You can also use the $this->session->sess_destroy() method to destroy the current session and remove all session data.

For more information about session management in CodeIgniter, you can refer to the official documentation: https://codeigniter.com/user_guide/libraries/sessions.html