login.php

<?php
require_once 'config.php';

session_start();

$error = '';

if (!(isset($_SESSION['login']) && $_SESSION['login']) && $_POST['login']) {

    $mysqli = new mysqli($config['host'], $config['login'], $config['password'], $config['db']);

    $username = $_POST['username'];
    $password = $_POST['password'];

    $result = $mysqli->query("select * from users where username='$username' AND password='$password'");

    $row = $result->fetch_row();

    if (count($row)) {
        $_SESSION['login_user'] = $username;
        $_SESSION['password_user'] = $password;
        $_SESSION['login'] = 'true';
        header("location: profile.php");
    } else {
        $error = "Username or Password is invalid";
    }

    $mysqli->close();
}