My php $_SESSION is not saving - php

Question

I'm starting with PHP, so I have no idea what's going wrong. I'm trying to update some value in my $_SESSION var using a function, but is not working.
First of all, I started the session and set a default value to the session if it is "undefined" in my header.php:
<?php
session_start();
if (!isset($_SESSION["locale"])) {
$_SESSION["locale"] = 'pt_BR';
}
?>
Then I have some ajax which calls my PHP function.
$('#change_locale_br').click(function() {
$.ajax({
type: "POST",
url: "i18n/i18n_functions.php",
data: { action: "changelocale", new_locale: "pt_BR" },
success: function() {
}
});
});
After in the php file, I have the following code:
<?php
function changelocale($l) {
$_SESSION["locale"] = $l;
}
if(isset($_POST['action']) && $_POST['action'] == 'changelocale'){
$l = $_POST['new_locale'];
changelocale($l);
}
?>
So the changelocale function is being called with the different values (pt_BR, es and etc), but after called it doesn't really change the $_SESSION["locale"] value.
I made a lot of research and NONE of the suggestions made here or in the PHP documentation page worked. Does someone knows what could be done in this case?
Thanks in advance.

Answer 1

You need to include session_start() at the top of your page, which you intend to set or access $_SESSION information.

Answer 2

Here is the solution: I called session_write_close(); (which is used to assure session data will be "unlocked", since php doesn't allow concurrent writes) right after setting the default values for the session in my header.php:
<?php
ini_set('session.cookie_domain', 'localhost');
if(!isset($_SESSION)) session_start();
if (!isset($_SESSION["locale"])) {
$_SESSION["locale"] = 'pt_BR';
}
session_write_close();
?>
Then in my changelocale.php I called the session_start() and at the end called the session_write_close(). It's like this now:
<?php
if(!isset($_SESSION)) session_start();
function changelocale($new_locale) {
$_SESSION["locale"] = $new_locale;
}
if(isset($_POST['action']) && $_POST['action'] == 'changelocale'){
$l = $_POST['new_locale'];
changelocale($l);
}
session_write_close();
?>