$username = $_POST['username'];
   $password = md5($_POST['password']);

   $db_mysq->query("SELECT * FROM <usertable>
                    WHERE <username>=\"".$username."\"
                    AND <password>=\"".$password."\"")
                    or die($db_mysq->error());
   
   if (mysql_num_rows($db_mysq->result) ==  1 ) {
   session_name('sid');
   session_start();
   //session_register($username);
   $_SESSION['username'] = $username;
   $_SESSION['time'] = time();
  }

session_name('sid'); 
 session_start();
 
 
 if(empty($_SESSION['username']))
 {
 /* user didn't got authorisation*/
  session_destroy();
  print("access denied");
 }
 else{
  setcookie("sid", session_id(), time()+ 18000 ); 
  .....
 }