access to webserver

Ok say one wanted to gain access to a webserver and they were allowed to upload files to that webserver but didn't know the pass or username… Is there a way of gaining that information or entry to the server through some sort of php file that can be uploaded to the server?

You can upload without logging in? How? And where can you upload to?

And you could upload some sort of shell/backdoor, but you'd have to be able to load that file.

FTP or HTTP server?