Run PHP process on receipt of file

This is more than likely OT, but I am hoping that someone has seen something like this. I am in of need something that will monitor a directory for file receipts and execute a PHP script on each file as it is received. A small daemon of some sort. I am STFW I we 'speak', but this group knows so much that A little more in-depth... We receive several hundred XML files each day that describe groups of services. I want/need to process each of those XML files in 'real-time' as they are received. For reasons too numerous to go into I cannot use a frequently running CRON job for this. Each file comes in via an FTP process started by the sender, I need to see the file when it is complete and then process it via the PHP script. So, the daemon would 'see' the completion of the file's arrival and would then call the appropriate PHP script. Can anyone point me in the right direction? TVMIA!