// set the template to be used as a string. {url} will be replaced automatically with the required url
$login->ActivationFunc = "Please click the following link to activate your account. <a href='{url}'>{url}</a>";
// OR a function with the first parameter being the url
$login->ActivationFunc = function ($url) {
    // use a template engine or do some action to generate the template (twig for example)
    $template = file_get_contents(__DIR__ . '/email_templates/activation.html');
    return str_replace("{url}", $url, $template);
};
// test your function like this: first parameter will be the activation url
//echo call_user_func($login->ActivationFunc, 'http://some_url');exit;
if (Crecket\AdvancedLogin\Core::$loggedIn !== false) {
    // check if use is logged in
    header('Location: index.php');
}
if (!empty($_POST['username']) && \SecureFuncs\SecureFuncs::getFormToken('register', $_POST['form_token']) !== false) {
    if ($login->register($_POST['username'], $_POST['email'], $_POST['password'], $_POST['repeat_password'])) {
        header('Location: index.php');
    }
}
$formToken = \SecureFuncs\SecureFuncs::setFormtoken('register');
?>
<!DOCTYPE html>
<html>
<head>
    <title>LoginScript - Register</title>
</head>
<body>
<div class="header">
    <pre>
<?php 
print_r($_SESSION[ADVANCEDLOGINSCRIPT_MESSAGE_KEY]);