how can I set a session variable in Drupal 8 and get it in a php script?

php session drupal session-variables

PHP Snippet 1:

$tempstore = \Drupal::service('tempstore.private')->get('your_module_name');
$tempstore->set('value', 'hi');

PHP Snippet 2:

$tempstore = \Drupal::service('tempstore.private')->get('your_module_name');
$some_data = $tempstore->get('value');