I am using CodeIgniter Framework 2.02

A Database Error Occurred
Error Number: 1054
Unknown column 'anil' in 'where clause'
SELECT user_id FROM users WHERE username = anil AND password = password
Filename: D:xampphtdocsc_loginsystemdatabaseDB_driver.php
Line Number: 330<

function check_login($username, $password)
  {
    $sha1_password = sha1($password);

    // The Guy uses ' ? ' in the video for the below statement and it works
    // I tried replacing ? with $username & $password, but it didnt work..
    // When I use the ?, I think it does query the DB, but I get the error
    // Incorrect Username or Password, Even though it is correct.
    // I only have 1 record on my DB, db = c_login, table=users
    // Fields are user_id   username    password    email   name

    $query_str = "SELECT user_id FROM users WHERE username = ? AND password = ?";
    $result = $this->db->query($query_str, $username, $sha1_password);

    if($result->num_rows() == 1)
    {
        return $result->row[0]->user_id;
    }
    else
    {
        return false;

    }
}

public function login()
    {
        $this->form_validation->set_rules('username', 'Username', 'required|trim|max_length[50]|xss_clean');
        $this->form_validation->set_rules('password', 'Password', 'required|trim|max_length[50]|xss_clean');

        // If form_validation has NOT been run, load the view login form
        if($this->form_validation->run() == FALSE)
        {
            $this->load->view('view_login');
        }
        else // Else process login
        {
            // Process input and login
            $username = $this->input->post('username');
            $password = $this->input->post('password');

            $user_id = $this->User_model->check_login($username, $password);

            if( ! $user_id)
            {
                // Login Failed Error
                $this->session->set_flashdata('login_error', TRUE);
                redirect('user/login');
            }
            else
            {
                // Log them in

                $this->session->set_userdata(array(
                                'logged_in' => TRUE,
                                'user_id' => $user_id
                                                    ));

                redirect('user/main_page');
            }

        }

    }

function check_login($username, $password)
  {
    $sha1_password = sha1($password);

    $this->db->select('user_id');
    $this->db->where('username', $username);
    $this->db->where('password', $sha1_password);
    $result = $this->db->get('users');

    if($result->num_rows() == 1)
    {
        $row = $result->row();
        return $row->user_id;
    }
    else
    {
        return false;
    }
}

if($user_id == FALSE)
  {
     $this->session->set_flashdata('login_error', TRUE);
     redirect('user/login', 'refresh');
  }
  else
  {
      $this->session->set_userdata(array(
                                       'logged_in' => TRUE,
                                       'user_id' => $user_id
                                    ));

      redirect('user/main_page', 'refresh');
  }

SELECT user_id FROM users WHERE username = anil AND password = password

SELECT user_id FROM users WHERE username = 'anil' AND password = 'password'

$result = $this->db->query($query_str, array($username, $sha1_password));