<form action="login.php" method="post" name="myForm" id="myForm">
    <input type="text" placeholder="Användarnamn..." name="username" id="username"><br />
    <input type="password" placeholder="Lösenord..." name="password" id="password"><br />
    <input type="submit" value="Spara ny användare" name="register" id="register"><br />
    <input type="submit" value="Logga in" name="login" id="login">
</form>

<?php

if(isset($_POST["register"])) {

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

    $userData = array("username" => $username, "password" => password_hash($password, PASSWORD_DEFAULT));

    $database = fopen("Database.txt", "w");

    fwrite($database, serialize($userData));
    fclose($database);
}

?>