if(isset($_POST['register'])) //  
{
    $user  = $_POST['username'];
    $password=$_POST['password'].PHP_EOL;
    $fh = fopen("file.txt","a+");

                   <form action="testinl2.php" method="POST">
                       
                       <p>Username <input type="text" name="username" id="namn" /></p>
                       <p>Password<input type="text" name="password" id="namn" /></p>
                       <input type="submit" name="Logg" id="Namn" value="Login"/> <!--Button-->