<?php

session_set_cookie_params(86400, '/', '...');
session_name('...');
session_start();

//It's about this session variable
//before having redirecting to this page, the Session variable had been set to -1
$_SESSION["questionAmount"]++;


if ($_SESSION["questionAmount"] == 3) {
    header('Location: /thankyou.php');
}

//connecting to database

$sql = "SELECT * FROM `questions` WHERE `id` = '".$_SESSION["questionAmount"]."';";

//executing $sql

//work something WITHOUT using or changing the session var

<form method="post" action="#" name="Formular" id="form_source">
                <fieldset>
                    <div id="question_wrapper">
                        <p id="question_question" style="display: none;"><?php echo $question; ?></p>
                    </div>
                    <p id="place"></p>
                    <div id="submit1_div" style="padding: 5px;">
                        <button id="submit1" name="answer" value="answer1" style="display: none;"><?php echo $answer1; ?></button>
                    </div>
                    <div id="submit2_div" style="padding: 5px;">
                        <button id="submit2" name="answer" value="answer2" style="display: none;"><?php echo $answer2; ?></button>
                    </div>
                    <div id="submit3_div" style="padding: 5px;">
                        <button id="submit3" name="answer" value="answer3" style="display: none;"><?php echo $answer3; ?></button>
                    </div>
                    <div id="submit4_div" style="padding: 5px;">
                        <button id="submit4" name="answer" value="answer4" style="display: none;"><?php echo $answer4; ?></button>
                    </div>
                </fieldset>
            </form>