I have class which is used to insert the 'id' in the table using PDO connection and now i have to store form data i.e id in session variable .

    <?php
    session_start();
    // this class is used to create connection with database
    class Database
    {
        private $db_host = ‘localhost’;
        private $db_user = ‘root’;
        private $db_pass = ‘root’;
        private $db_name = ‘test’;   

        public function connect()   {
          $db = new PDO('mysql:host=$db_host;dbname=$db_name;charset=utf8mb4', '$db_user', '$db_pass');
          }
    }

  this class is used to insert the id in the table 

    class table1 extends Database
    {
           public function insert_info()
            {
                    $sql = "insert into info(id) values ('?')";
                    $sql->bind_param("s", $id);
                    $sql->execute();
                    return true;
            }
    }

    $_SESSION['campid']='camp1001';
    $db = new table1();       // it is used to object of class table1.
    $res=$db->insert_info();

    ?>

How to store session variable in table how it will be achieved .

Why not just pass the $_SESSION['campid'] value to insert_info() method?

$_SESSION['campid']='camp1001';
$db = new table1();       // it is used to object of class table1.
$res=$db->insert_info($_SESSION['campid']);

and the function

class table1 extends Database
{
       public function insert_info($id) // <-- add function param
        {
                $sql = "insert into info(id) values ('?')";
                $sql->bind_param("s", $id);
                $sql->execute();
                return true;
        }
}