This function is often cited related to file upload tracking with PHP 5.2. So I thought this is a good place to put a warning.
If you setup PHP with FastCGI, you'll probably run into trouble using this function to get any information about a running upload. At least in my case, every HTTP request is handled by a different PHP process. I could track it with the getmypid() function, which returned a different value upon every request, but only from a limited set. Also, apc_cache_info() gave me all upload_* entries that were created in that process. So when the upload was initially catched by one PHP process, all progress updates must be fetched from the same process, too, because APC cache information does not seem to be shared across multiple processes handling that domain/virtual host. But that's impossible to tell because PHP has its own load management and serves every request by an arbitrary process.
So in short: When using FastCGI and multiple PHP processes (recommended for performance reasons), you cannot use APC upload tracking. You'll only get a status update every few requests.