Refer to a global variable doesn't work - php

Question

A im writing a simple script in PHP with few files, in 'loginForm.php' I have code:
<?php
session_start();
$login = $_GET['login'];
$password = $_GET['password'];
$remember = $_GET['remember'];
include_once("login.php");
$userAccount = new UserAccount($login, $password);
$logged = $userAccount -> isLogged();
(...)
and in file 'photo.php':
global $userAccount;
$login = $userAccount -> getLogin();
what gives mi an error:
Call to a member function getLogin() on a non-object
I also tried with $GLOBALS - same result.

Answer 1

Global variable does not work across requests, but have file scope.
the typical use of global variables if to have a variable that is accessible across different scopes (normally functions in the same file.
For example in file1.php
<?php
$value = 1;
echo $value; // prints '1'
function f1() {
global $value;
$value++;
}
echo $value; // prints '2'
function f1() {
$value++;
}
echo $value; // prints '2'
?>
To use variables across requests, use sessions.
safest way to create sessions in php
Storing objects in PHP session

Answer 2

OK, now i did:
1) In AJAX-request file i create a instance of Class UserAccount, which will be store in $_SESSION array
<?php
include_once("login.php");
session_start();
$login = $_GET['login'];
$password = $_GET['password'];
$rememberMe = $_GET['remember'];
$userAccount = new UserAccount();
$userAccount -> LogIn($login, $password);
$logged = $userAccount -> isLogged();
$_SESSION['userAccountClassObject'] = serialize($userAccount);
2) In static (non AJAX-request) file 'photo.php' it works fine:
<?php
include_once("login.php");
$user = unserialize($_SESSION['userAccountClassObject']);
$login = $user -> getLogin();
3) But in other AJAX-request file - addComment.php unfortunatelly doesn't work:
<?php
$id = $_GET['id'];
$comment = $_GET['comment'];
session_start();
include("login.php");
$user = unserialize($_SESSION['userAccountClassObject']);
$login = $user -> getLogin(); // Fatal error