PHP session variable empty during upload - php

Question

PHP states:
When the session.upload_progress.enabled INI option is enabled, PHP
will be able to track the upload progress of individual files being
uploaded.
I check mine beforehand: echo ini_get("session.upload_progress.enabled");, returns 1 (true)
The upload progress will be available in the $_SESSION superglobal
when an upload is in progress, and when POSTing a variable of the same
name as the session.upload_progress.name INI setting is set to.
When PHP detects such POST requests, it will populate an array in the
$_SESSION, where the index is a concatenated value of the
session.upload_progress.prefix and session.upload_progress.name INI
options.
Code:
<?php
session_start();
?>
<form action="upload.php" method="POST" enctype="multipart/form-data">
<input type="hidden" name="<?php echo ini_get("session.upload_progress.name"); ?>" value="123" />
<input type="file" name="file1" />
<input type="file" name="file2" />
<input type="submit" />
</form>
<?php
echo '<pre>';
print_r($GLOBALS);
echo '</pre>';
?>
Result:
Array
(
[_GET] => Array
(
)
[_POST] => Array
(
[PHP_SESSION_UPLOAD_PROGRESS] => 123
)
[_COOKIE] => Array
(
[PHPSESSID] => pmbqca8fedqgg3hg6nge41fno2
)
[_FILES] => Array
(
[file1] => Array
(
[name] => profile.jpg
[type] => image/jpeg
[tmp_name] => /Applications/XAMPP/xamppfiles/temp/phpPLwrND
[error] => 0
[size] => 6946
)
[file2] => Array
(
[name] =>
[type] =>
[tmp_name] =>
[error] => 4
[size] => 0
)
)
[GLOBALS] => Array
*RECURSION*
[_SESSION] => Array
(
)
)
The SESSION variable is completely empty. Why is this?