Well, I'm running PHP 5.2 on my computer, and it works well. I have an image hosting script on my computer and it works fantastic under my Apache setup, however when I upload it to my host, it doesn't work.
The main problem is renaming things from $_FILES['file']['name'] to $file['name'], etc. to make it use more friendly variables.. does PHP 5.1.4 differ that greatly from PHP 5.2?