PHP, Unix MV command and users

Hi there guys, I have a website for Digital Photo Printing and my users send the pictures through a JAVA FTP Applet.
Under my PHP script, I would like to be able to use “MV” command in order to move the files (pictures) from the FTP subdirectory to a secure directory of my website, where I will control the picture and save the info to a mysql database.
The problem is that I can´t find a way of moving the files from the FTP subdirectory with the “mv” command with my script user (I always get a “you do not have the right to do that” kind of message).
I can´t just change the “run as userx” of my website, because I have more than one ftp user. And all my users are under the same group.
The only solution I could find so far, is to use PHP FTP commands to do the task, but of course it is slower.
Can anyone guess anything about this? Has anyone had such a problem?