Import files from directory

Hi everybody, I am running a real estate portal and would like to allow users to upload their listing from their existing software. They do this with a XML file which they will upload on a ftp server that places this xml file into a seperate directory on the file system. My basic idea was to run a php scipt triggered by cron every 5 minutes that checks if there is a new upload and then reads the file and removes it. Now here is where the problem starts. Imagine if there are 1000 users, I would need to go through 1000 folders to check for new content. There must be a better solution to identify the directory that has a new finished upload. Has anybody an idea on how to do this? Thank you for any hint. Best regards, Merlin