I'm writing some HTML parser for my website because most of the contents are retrieved from a remote location. I construct the remote URL internally and retrieve the remote contents using file_get_contents().
When I'm testing the website from different computers, the setting changes made by one computer affects the settings of the rest. So I believe regardless of the end users, the retrieval process is done by the web hosting server with the same IP address, and therefore the remote server recognizes all of us as the same person.
So it seems you need to show different content to different users, to achieve that your application should process information like the end users IP, browser, OS, and the end user's settings saved via cookie in his browser if that user is not a registered member, if the user is a registered member his settings can be saved somewhere in a database.
You mentioned file_get_contents() and I understood that you use PHP. Well, there's just one PHP executable on the server, processing all user scripts, and that's the thing that actually retrieves content. So the sites that are being retrieved "see" only that PHP executable and its address.
Little can be done here... You may try to work through different proxies. PHP itself cannot work with proxies, but the CURL extension can. RTFM