$KEY = "something really long long long long long and secret";
$email = $_POST['email'];
$user = get_user_by_email($email);
if ($user && $user['id'])
{
    $time = time();
    $hash = md5( $user['id'] . $time . $KEY);
    $url = "http://domain.com/reset-password-form.php?id=".$user['id'].'&timestamp='.$time.'&hash='.$hash;
    send_email($email, 'reset password email from xxx.com', ' Please click the following link to reset password'. $url);
}

$KEY = "something really long long long long long and secret";
$hash = $_GET['hash'];
$user_id = $_GET['id'];
$timestamp = $_GET['timestamp'];

if ($hash == md5( $user_id . $timestamp . $KEY ))
{
    if ( time() - $timestamp > 3600 ) // one hour
    {
        die('link expired');
    }
}
else
{
    die('invalid parameters');
}

//validation passed

if ($_POST['new_password'])
{
    reset_user_password($user_id, $_POST['new_password']);
    die(' password changed successfully ');
}
else
{
    echo '
        <form action="reset-password-form.php?hash=$hash&id=$user_id&timestamp=$timestamp" method="post">
            new password: <input type="password" name="new_password">

            <input type="submit" value="submit">
        </form>
    ';
}