When the session.upload_progress.enabled INI option is enabled, PHP will be able to track the upload progress of individual files being uploaded. This information isn't particularly useful for the actual upload request itself, but during the file upload an application can send a POST request to a separate endpoint (via XHR for example) to check the status.

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. The key is typically retrieved by reading these INI settings