Why would $_FILES be empty when uploading files to PHP?

Question

I have WampServer 2 installed on my Windows 7 computer. I\'m using Apache 2.2.11 and PHP 5.2.11. When I attempt to upload any file from a form, it seems to upload, but in PH

Answer 1

Thank you everybody for the vary comprehensive replies. Those are all very helpful. The answer turned out to be something very odd. It turns out that PHP 5.2.11 doesn't like the following:

post_max_size = 2G

or

post_max_size = 2048M

If I change it to 2047M, the upload works.