Halfabee is right.
PHP is spawned or forked by the web service - so the UID of the PHP process would typically be:
in UN*X:
nobody, httpd, apache, wheel
in Win32
where MACHINENAME is your system's hostname.
This is not a bug but a security feature. You can use certain "sudo/chown" commands to overcome this but this will only break a bigger hole in an already open system (web server).
Also make sure your uploaded files are placed in a "safe" directory. You don't want users posting their own PHP/CGI codes up there and then run them.