https://docs.joomla.org/JFactory/getUserCodice PHP:
$user = JFactory::getUser();
if (!$user->guest) {
echo 'You are logged in as:<br />';
echo 'User name: ' . $user->username . '<br />';
echo 'Real name: ' . $user->name . '<br />';
echo 'User ID : ' . $user->id . '<br />';
}