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Session Library Documentation CodeIgniter 3.x <http://codeigniter.com/userguide3/libraries/sessions.html>_
:doc:Session Library Documentation CodeIgniter 4.x </libraries/sessions>

What has been changed

Only small things like the method names and the loading of the library have changed.
The definition of the session table in Database Driver has changed.

Wherever you use the Session Library replace $this->load->library('session'); with $session = session();.
From that on you have to replace every line starting with $this->session with $session followed by the new method name.
- To access session data use the syntax $session->item or $session->get('item') instead of the CI3 syntax $this->session->name.
- To set data use $session->set($array); instead of $this->session->set_userdata($array);.
- To remove data use unset($_SESSION['some_name']); or $session->remove('some_name'); instead of $this->session->unset_userdata('some_name');.
- To mark session data as flashdata, which will only be available for the next request, use $session->markAsFlashdata('item'); instead of ``$this->session->mark_as_flash('item');```
If you use Database Driver, you need to recreate the session table. See :ref:sessions-databasehandler-driver.