index.php

<?php

    $error = "";

        if(isset($_POST['submit'])) {

            $query = mysqli_query($this->db,"SELECT * FROM users
                WHERE username = '".$_POST['username']."' AND password = '".$_POST['password']."'");

            if(mysqli_num_rows($query) == 0) {
                $error = "Username or Passowrd is incorrect.";
            } else {
                $fetch = mysqli_fetch_array($query);
                $_SESSION['user'] = $fetch['id'];
                $error = "You are now logged in.";
            }

        }