Parse.com PHP doesn't keep user in session after signup or login

So I'm using Parse.com with PHP for my user system but it doesn't work as expected. When I call the signUp() method the new user is successfully saved in Parse DB and is logged in but if I navigate to another page ParseUser::getCurrentUser() doesn't seem to fetch the current user. The same thing happens with the ParseUser::logIn() method. Here's part of my code:

<?php

            require 'vendor/autoload.php'; 

            use Parse\ParseClient;

            ParseClient::initialize('*, *, *');

            use Parse\ParseUser;
            use Parse\ParseException;
            use Parse\ParseSessionStorage;

            function showForm() {
                echo "
                <form action='user.php' method='POST'>
                    Όνομα (χωρίς #)<br />
                    <input type='text' name='name' required><br /><br />
                    Κωδικός πρόσβασης<br />
                    <input type='password' name='password' required><br /><br />
                    <input type='submit' name='login' value='Σύνδεση'> <input type='submit' name='register' value='Εγγραφή'>
                </form>
                ";
            }
            if ($_SERVER['REQUEST_METHOD'] == "POST") {
                if (isset($_POST['login'])) {
                    try {
                        $user = ParseUser::logIn($_POST['name'], $_POST['password']);
                        echo "<a href='add.php'>Continue</a>";
                        $currentUser = ParseUser::getCurrentUser();
                        echo $currentUser->getUsername();
                    } catch (ParseException $error) {
                        echo "Error: " . $ex->getCode() . " " . $ex->getMessage();
                        showForm();
                    }
                } else if (isset($_POST['register'])) {
                    $user = new ParseUser();
                    $user->set("username", $_POST['name']);
                    $user->set("password", $_POST['password']);

                    try {
                        $user->signUp();
                        echo "Εγγραφήκατε επιτυχώς!";
                        $currentUser = ParseUser::getCurrentUser();
                        echo $currentUser->getUsername();
                    } catch (ParseException $ex) {
                        echo "Error: " . $ex->getCode() . " " . $ex->getMessage();
                        showForm();
                    }
                }
            } else {
                showForm();
            }
        ?>