Query regarding temporarily-uploaded files

Hi all. I have a requirement, wherein I need to allow vanilla uploads of files to a HTTPD server. Any client can upload any number of files (one at a time). Also, there is just one directory, where the files get stored "finally" (that is, after being copied from the temporary location, via "move_uploaded_file") Also, I have been able to get the simple file uploading running via PHP, by picking up one of the numerous "Hello World" examples available :) Now, I am facing the following use-case :: 1) User 1 starts uploading a large file, say "big_file.avi". 2) Meanwhile, user 2 also starts uploading a (different) file, but with the same name "big_file.avi". In an ideal scenario, user 2 should be prompted with a message, that a file of the same name is already being uploaded by someone else somewhere. Is there a way to do this? ( Note that, had the user 2 started uploading AFTER user 1 had finished with the upload, we could probably modify the PHP-script at the sever-side, to let user-2 know that a file of the same name already exits. But I am failing to find a solution, when the user 2 starts the upload WHILE the large file of user 1 is in the process of completing uploading). Any way the issue may be solved? I will be grateful for any pointers :) Regards, Ajay