I'm busting my mind towards the wall .... I'm creating a simple joomla module, in assistant.php I cant assign values published in the form.

<?php

// no direct access
defined('_JEXEC') or die('Restricted access');

class modReservationHelper {
    public $name;
    public $email;
    public $message;
    public $comment;

    protected function  __construct() {
        $this->name = $_POST['fullname'];
        $this->email = $_POST['email'];
        $this->message = $_POST['message'];
        $this->comment = $_POST['comment'];
    }

     function validateForm() {
        echo $this->name;   //The output is always 0
        echo $this->email+"</br>";//The output is always 0
        echo $this->message;//The output is always 0


        //When I try
        echo $_POST['comment']; // Is correct
        }   
    }

?>

Also I have attempted to not use constructor with similar Zero effect:(

<?php

// no direct access
defined('_JEXEC') or die('Restricted access');

class modReservationHelper {
    public $name;
    public $email;
    public $message;
    public $comment;


    function  getValues() {
        $this->name = $_POST['fullname'];
        $this->email = $_POST['email'];
        $this->message = $_POST['message'];
        $this->comment = $_POST['comment'];
    }

     function validateForm() {
        modReservationHelper::getValues;
        echo $this->name;   //The output is always 0
        echo $this->email+"</br>";//The output is always 0
        echo $this->message;//The output is always 0

        //When I try
        echo $_POST['comment']; // Is correct
        }   
    }

?>

The entire procedure is known as from "mod_wreservation.php" I call modReservationHelper::validateForm()

You're calling the course in static manner. So $this at school won't be object of modReservationHelper.

The proper way to make use of this as with mod_wreservation.php is

$helperObj = new modReservationHelper(); // your choice will work (__counstruct) with this
$helperObj->validateForm();

For second choice

$helperObj = new modReservationHelper();
$helperObj->setValue();
$helperObj->validateForm();

and sophistication is going to be

<?php

// no direct access
defined('_JEXEC') or die('Restricted access');

class modReservationHelper {
    public $name;
    public $email;
    public $message;
    public $comment;


    function  setValues() {
        $this->name = $_POST['fullname'];
        $this->email = $_POST['email'];
        $this->message = $_POST['message'];
        $this->comment = $_POST['comment'];
    }

     function validateForm() {            
        echo $this->name;   //The output is always 0
        echo $this->email+"</br>";//The output is always 0
        echo $this->message;//The output is always 0

        //When I try
        echo $_POST['comment']; // Is correct
        }   
    }

?>

and it'll be superior if you are using this in mod_wreservation.php

$post = JRequest::get('post');
$helperObj = new modReservationHelper();
$helperObj->setValue($post);
$helperObj->validateForm();