php - Twig with MVC - how to get variable after run action file

I'm building my PHP project with MVC and Twig and I'm having problem with return one variable string after form validation on action 'input'. Here is my controller file:

// index.php file - here is the begin

require_once './vendor/autoload.php';
//require_once "autoloader.php";
require_once "class/blogcore.php";
require_once "class/pagination.php";
require_once "class/post.php";
require_once "class/validation.php";

$conf = parse_ini_file("config.ini");
$lang = parse_ini_file($conf['LANG_PATH']);
$db = new PDO('mysql:host='.$conf['DB_HOST'].';dbname='.$conf['DB_NAME'].';charset='.$conf['DB_CHARSET'], $conf['DB_USER'], $conf['DB_PASS']);

/*
** This is the controller file
*/

$action = !empty($_GET['action']) ? $_GET['action'] : 'Index';
require_once 'actions/' . $action . '.php';

$action = new $action($db, $lang);
$data = $action->generate();
$template = $action->getTemplate();

if ($template) {
    $loader = new Twig_Loader_Filesystem('templates');
    $twig = new Twig_Environment($loader);
    echo $twig->render($template, $data);   
}
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Here is myindex.php file which is displaying the index page:

// '/class/index.php' file with class called 'Index'

class Index {
    private $lang;
    private $db;

    public function __construct($db, $lang) {
        $this->lang = $lang;
        $this->db = $db;
    }

    public function generate() {

        // new Core object 
        $core = new BlogCore($this->db);

        /*
        ** pagination
        */
        $numRows = $core->getNumRows();
        $pagination = new Pagination($_GET['p']);
        $pagination->setNumRows($numRows);
        $pagination->setLimit(3); // set 3 posts on one page
        $limit = $pagination->sqlPagination();
        $offset = $pagination->sqlPaginationOffset($limit);

        /*
        ** select detalis form database
        */
        $posts = $core->select($limit, $offset);

        return [
                "posts"         => $posts,
                "pagination"    => $pagination,
                "lang"          => $this->lang
                ];
    }

    public function getTemplate() {
        return "default.tpl";
    }
}
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And here is theinsert.php file which is used on actioninsert (just when I want do input new data from form to my db). Here I'm checking with regular expression that data filled in fields are correctly. And if not, here I want to return to template my variable which will be displaying notification for user.

// '/class/insert.php' file with 'Insert' class

class Insert {
    private $lang;
    private $db;
    private $postText;
    private $fileName;
    private $myFile;
    private $phoneValidate;
    private $urlValidate;

    public function __construct($db, $lang) {
        $this->lang = $lang;
        $this->db = $db;
    }

    public function generate() {

        $postText   = $_POST["postText"];
        $myFile     = $_FILES["myFile"];
        $fileName   = $myFile["name"];
        $phoneNum   = $_POST['phoneNumber'];
        $url        = $_POST['url'];

        $validate   = new Validation;

        $phoneValidate = "test";
        // phoneValidate for form validation
        if (!$validate->phoneNum(isset($phoneNum))) {
            $phoneValidate = "error";
        } else {
            $phoneValidate = "ok";
        }

        // new Core object
        $core = new BlogCore($this->db);

        // do insert in SQL
        if (!empty($postText)) {
            $core->insert($postText, $fileName);
            $core->saveFile($myFile);
        }

        header("Location: /codeme/06_blog_v5_mvc/index.php");
    }

    public function getTemplate() {
        return null;
    }
}
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So the question is: how can I modify this file to get my index file (like I'm doing this actually withheader() function and get additional value of variable called$phoneValidate? If You need, this project is shared on my BitBucket account: https://bitbucket.org/Pirum/simple-blog/src