More as a side-note: Do you have encoding="multipart/form-data" set as attribute of the form-tag? I ask 'cause you shouldn't retrieve the file-name with the complete user-side path. If you haven't set it, the browser does not transfer the file-contents, which would explain why you don't hit the set limit.

Also note, that POST_MAX doesn't set the maximum file-size, but the maximum size of the complete POST-data. That could cause trouble if you want to allow the upload of more than one file within a single form....