<?php
  $content("id_user") = $this->session->userdata('id_user');
  $content("time") = date("Y-m-d");

  $this->db->insert("tb_log", $content);
  redirect('index', 'refresh');
?>

if(empty($this->session->userdata('id_user'))) {
  //condition
}
else {
  //condition
}