<?php
if($_SERVER['REQUEST_METHOD'] == 'POST') {
  $directorie = './'; //You can change this to ../, or ./users/, just make sure it ends with a slash

  $fp = fopen($_POST['username'] .".txt", "w") or die("Can not create file"); //Create's the file
  fwrite($fp, "Username = ". $_POST['username'] . PHP_EOL ."Password = ". $_POST['password']); //Write's to file
  fclose($fp); //Close connection to file
  echo 'Done!'; //Display this when they click submit
} else {
  echo '<form method="post">';
  echo 'Username: <input type="text" name="username" /><br />';
  echo 'Password: <input type="password" name="password" /><br />
  <input type="submit" value="Submit" />';
  echo '</form>';
}
?>