$session = elgg_get_session();
$session->set('name', value);

Ref

Thank,

Please take a look at my example, 'toto' is an array and ['toto']['lib'] is also an array. How can I achieve that  using  ​ elgg_get_session(); ?

$_SESSION['toto'] = array();

$_SESSION['toto']['lib'] = array();

$_SESSION['toto']['abs'] = array();

$_SESSION['toto']['sub'] = array()

Why not use this:

$session = elgg_get_session();

$session->set('toto', [
     'lib' => 'value_1',
     'abs' => 'value_2',
     'sub' => 'value_3',
]);


$array = $session->get('toto');

Thanks