function random_str($length = 10, $keyspace = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'){
    $str = '';
    $max = mb_strlen($keyspace, '8bit') - 1;
    for ($i = 0; $i < $length; ++$i) {
        $str .= $keyspace[random_int(0, $max)];
    }
    return $str;
  }

  public function change_password($token){
    if(isset($this->session->userdata['logged_in'])) {
      redirect('dashboard');
    }
    $user_info = $this->User_model->get_info_from_reset_token($token);
    if(!isset($user_info)){
      show_404();
      die();
    }
    $username = $user_info->username;
    $timestamp = $user_info->timestamp;

    $this->session->set_flashdata('username',$username);

    $time = new DateTime($timestamp, new DateTimeZone('UTC'));
    $res_time = strtotime($time->format('Y-m-d H:i:s O'));
    $now_time = time();
    if(($now_time-$res_time)<86400){
      $this->load->view('auth/set_password');
    }
    else {
      $data = array(
        'error_msg' => "Reset link has expired. Enter username to send again."
      );
      $this->load->view('auth/reset_password',$data);
    }
  }

  public function set_reset_password(){
    if(isset($this->session->userdata['logged_in'])) {
      redirect('dashboard');
    }
    $password = $this->input->post('password');

    $this->User_model->update_password($password,$_SESSION['username']);
    $this->session->unset_userdata['username'];
    $this->load->view('auth/reset_pass_done');
  }

  public function logout(){
    $this->session->unset_userdata('logged_in');
    redirect('auth');
  }
}


---------------------------------Model----------------------------------------------

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class User_model extends CI_Model {
	public function __construct() {
		parent::__construct();
	}

	public function check_user($username) {
		$this->db->from('users');
		$this->db->where('username',$username);
		$query = $this->db->get();
		$row = $query->row();

		if(isset($row)){
			return true;
		}
		return false;
	}

	public function get_password($username){
		$this->db->from('users');
		$this->db->where('username',$username);
		$query = $this->db->get();

		$row = $query->row();
		return $row->password;
	}

	public function get_user_data($username) {
		$this->db->from('users');
		$this->db->where('username',$username);
		$query = $this->db->get();
		$row = $query->row();

		$data = array(
			'username' => $username,
			'firstName' => $row->first_name,
			'lastName' => $row->last_name,
			'role' => $this->get_role($username),
			'dept' => $this->get_user_dept($username),
		);

		return $data;
	}

	public function get_username($data) {
		$this->db->select('user_role.user_name');
		$this->db->from('user_role');
		$this->db->join('user_department', 'user_role.user_name = user_department.user_name');
		$this->db->where($data);
		$query = $this->db->get();

		$row = $query->row();

		return $row->user_name;
	}

	public function get_role($username){
		$this->db->from('user_role AS A');
		$this->db->join('roles AS B','A.role_id = B.role_id');
		$this->db->where('user_name',$username);
		$this->db->select('A.role_id, role_name');
		$query = $this->db->get();
		$row = $query->row();
		return $row;
	}

	public function get_user_dept($username){
		$this->db->from('user_department AS A');
		$this->db->join('departments AS B','A.department_id = B.department_id');
		$this->db->where('user_name',$username);
		$this->db->select('B.department_id, B.department_name');
		$query = $this->db->get();
		$row = $query->row();

		return $row;
	}

	public function get_user_email($username){
		$this->db->select('email');
		$this->db->from('users');
		$this->db->where('username',$username);
		$query = $this->db->get();

		$email = $query->row()->email;
		return $email;
	}

	public function get_user_first_name($username){
		$this->db->select('first_name');
		$this->db->from('users');
		$this->db->where('username',$username);
		$query = $this->db->get();

		$firstName = $query->row()->first_name;
		return $firstName;
	}

	public function add_password_reset($username,$token){
		$data = array(
			'username' => $username,
			'token' => $token
		);
		$this->db->insert('password_resets',$data);
	}

	public function get_info_from_reset_token($token){
		$this->db->from('password_resets');
		$this->db->where('token',$token);
		$query = $this->db->get();

		$row = $query->row();
		return $row;
	}

	public function in_reset_password($username){
		$this->db->from('password_resets');
		$this->db->where('username',$username);
		$query = $this->db->get();

		if($query->num_rows()==0){
			return false;
		}
		return true;
	}

	public function update_password($password,$username){
		$user = $username;
		$this->db->where('username',$user);
		$this->db->update('users', array('password' => $password));

		$this->db->delete('password_resets', array('username' => $user));
	}
}