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, i.e. ]]> It is also possible to cancel the currently in-progress file upload, by setting the $_SESSION[$key]["cancel_upload"] key to &true;. When uploading multiple files in the same request, this will only cancel the currently in-progress file upload, and pending file uploads, but will not remove successfully completed uploads. When an upload is cancelled like this, the error key in $_FILES array will be set to UPLOAD_ERR_EXTENSION. The session.upload_progress.freq and session.upload_progress.min_freq INI options control how frequent the upload progress information should be recalculated. With a reasonable amount for these two settings, the overhead of this feature is almost non-existent. Example of the structure of the progress upload array. " value="123" /> ]]> The data stored in the session will look like this: 1234567890, // The request time "content_length" => 57343257, // POST content length "bytes_processed" => 453489, // Amount of bytes received and processed "done" => false, // true when the POST handler has finished, successfully or not "files" => array( 0 => array( "field_name" => "file1", // Name of the field // The following 3 elements equals those in $_FILES "name" => "foo.avi", "tmp_name" => "/tmp/phpxxxxxx", "error" => 0, "done" => true, // True when the POST handler has finished handling this file "start_time" => 1234567890, // When this file has started to be processed "bytes_processed" => 57343250, // Number of bytes received and processed for this file ), // An other file, not finished uploading, in the same request 1 => array( "field_name" => "file2", "name" => "bar.avi", "tmp_name" => NULL, "error" => 0, "done" => false, "start_time" => 1234567899, "bytes_processed" => 54554, ), ) ); ]]> The web server's request buffering has to be disabled for this to work properly, else PHP may see the file upload only once fully uploaded. Servers such as Nginx are known to buffer larger requests. The upload progress information is written to the session before any scripts are executed. Therefore changing the session name via ini_set or session_name will give a session without the upload progress information.