Poll a Folder for new file

Hi All - hope someone has done this before.

I need to create a PHP script that detects the presence of a new file appearing into a folder, and process it.

This HAS to be supported by Windows IIS (Have looked at FAM for Linux but can't use it).

I could do a loop with some sleep that checks the directory every x seconds - but this has been discounted by the client as either a) too resource hungry or b) not quick enough if there is a delay.

Add ons, extensions or even external 3rd party tools would all be acceptable.

Any ideas?

Phil