(3.0.17 - 3.0.18 only, PHP 4 >= 4.0.3)is_uploaded_file -- Tells whether the file was uploaded via HTTP POST.
This function is available only in versions of PHP 3 after PHP 3.0.16, and in versions of PHP 4 after 4.0.2.
Returns TRUE if the file named by filename was uploaded via HTTP POST. This is useful to help ensure that a malicious user hasn't tried to trick the script into working on files upon which it should not be working--for instance, /etc/passwd.
This sort of check is especially important if there is any chance that anything done with uploaded files could reveal their contents to the user, or even to other users on the same system.