Note, this feature doesn't work, when your webserver is runnig PHP via FastCGI. There will be no progress informations in the session array.
Unfortunately PHP gets the data only after the upload is completed and can't show any progress.

I hope this informations helps.

ATTENTION:

Put the upload progress session name input field BEFORE your file field in the form :

  <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>

If you make it after your file field, you'll waste a lot of time figuring why (just like me ...)

The following form will make you crazy and waste really a lot of time:

<form action="upload.php" method="POST" enctype="multipart/form-data">
 <input type="file" name="file1" />
 <input type="file" name="file2" />
 <input type="hidden" name="<?php echo ini_get("session.upload_progress.name"); ?>" value="123" />
 <input type="submit" />
</form>

DON'T do this!

While the example in the documentation is accurate, the description is a bit off. To clarify:

PHP will populate an array in the $_SESSION, where the index is a concatenated value of the session.upload_progress.prefix and the VALUE of the POSTed session.upload_progress.name variable.

If you're seeing
"PHP Warning:  Unknown: The session id is too long or contains illegal characters, valid characters are a-z, A-Z, 0-9 and '-,' in Unknown on line 0",
then a misplaced input could be the cause. It's worth mentioning again that the hidden element MUST be before the file elements.

Note that if you run that code and you print out the content of $_SESSSION[$key] you get an empty array due that session.upload_progress.cleanup is on by default and it  cleans the progress information as soon as all POST data has been read.

Set it to Off or 0 to see the content of $_SESSION[$key].

it should be noted that the hidden element come before the file element otherwise you wont get any updates.

dont't forget, that the session has to be initialized before the form is generated, otherwise the mentioned example above won't work.

Warning:
Do not change session.save_path and session.name (in your application)!

The request for upload info must by POST with same value and name of your hidden input (session.upload_progress.name).

Info:
It works under IIS /FastCGI (at PHP 5.4 and PHP 8.2, other not tested).

There were two gotchas that got me with implementing this.

The first - if you use session_name() to change the name of sessions, this will not work. I discovered this by looking at phpinfo() and seeing that is saw a different session name.

At least in Apache, a better way to set the session is in your apache config use

php_value session.name "your custom name"

It goes within the Directory directive, might work in .htaccess - I don't know.

-=-

Secondly - in apache, don't use mod_mpm_prefork.so

That was the problem I had, that's the default in CentOS 7.

The problem is it causes Apache to wait with any additional requests until the upload is finished.

Commenting that module out and using mod_mpm_worker.so instead fixed that problem, and the progress meter worked.

session.upload_progress updates completely ignore custom session handlers set via  session_set_save_handler()

It seems like if you send a form with the field like :

<?php echo '<input type="hidden" name="'.ini_get('session.upload_progress.name') .'" value="123" />';  ?>

without any field type "file", the server respons will be an 500 error.