how can i use set_userdata in codeigniter4 for adding session data

Suggesting you as per CI doc. Initialize session by

$session = \Config\Services::session($config); // $config is optional here

then,

$newdata = [
        'username'  => 'johndoe',
        'email'     => '[email protected]',
        'logged_in' => TRUE
];

$session->set($newdata);

Or

$session->set('some_name', 'some_value');

Let me try to explain to you the easy way. In CI4 you will need to initialized the session library

In Controller

$session = \Config\Services::session();
$sessionData = [
    'username' => 'John Doe',
    'phone' => 123545678,
    'role' => 'Teacher',
    'status' => 'Active'
];
$session->set($sessionData);

To Read the Data it is as simple as

echo $session->get('username');

For More information just visit the documentation

From the doc,

You can simply assign data to the $_SESSION array, as with any other variable. Or as a property of $this->session.

Alternatively, the old method of assigning it as “userdata” is also available. That however passing an array containing your new data to the set_userdata() method.

it looks like you can use the set command with an array.

$newdata = [
        'username'  => 'johndoe',
        'email'     => '[email protected]',
        'logged_in' => TRUE
];

$session->set($newdata);

or use it like that to set a single value

$session->set('some_name', 'some_value');

Hello if I can understand your question. You want to set a session data. Here is what you will do to solve this particular issue. Load the session library by using any these session() or service('session')

$session = session();
$session->set('data_index', $data);

Reference the document CI4 Doc

You can set session value in Codeigniter4 as:

session()->set('session_name','session_value');