Upload Progress: Track file upload progress

This class can be used to track file upload progress.

It can generate an identifier to track that the file upload that should be used in the HTML form as a hidden field.

When the upload form is submitted, PHP can associate the file being uploaded with the generated identifier, so the class can retrieve information of the upload progress in a parallel HTTP request eventually sent via AJAX.

The class uses the PHP upload progress extension which can be configured to keep track of the upload progress using an APC cache entry, PHP session variables or PHP temporary files.