<?php
#1.create map with the name users
#2 create username.txt
#3.add username and password in username.txt like this example:"username ~~ password"
# form with input's included
session_start();
if (isset($_GET['submit'])) {
  $username = $_GET['username'];
  $password = $_GET['password'];
  $path = './users/'. $username . '.txt';
  if (file_exists($path)) {
    $userfile = file_get_contents($path);
    $checker = explode('~~', $userfile);
    if ($password == $checker[1]) {
      $_SESSION['username'] = $username;
      # script na succesfull login
    } else { echo '<script type="text/javascript">alert("Password incorrect!")</script>';}
  } else { echo '<script type="text/javascript">alert("Username incorrect!")</script>';}
} else #submit error
 ?>

    <form class="login" action="login.php" method="get">
        <h1>Login</h1>
        <input type="firstname" name="username" value="" placeholder="Username..." required autofocus> <br />
        <input type="password" name="password" value="" placeholder="Password..." required ><br />
        <button type="submit" name="submit">Submit</button><br />
    </form>