You could use something similar to these examples...
http://www.example.com/?user1
print $QUERY_STRING; // outputs 'user1'
...or....
http://www.example.com/index.php/user1
print $PATH_INFO; // outputs '/user1'
You could rename 'index.php' to 'users' and force your webserver to parse the file 'users' using PHP, this makes it look a lot neater.
http://www.example.com/users/user1
print $PATH_INFO; // still outputs '/user1'
Apart from that, I dont know of any other ways.
Also, PATH_INFO or QUERY_STRING might only be available on specific webservers? (I know they are defined under Apache). But there should be an equivalent on others.