A second question about http File Upload...

In my previous message I stated how some uploads were failing past a certain size. What I want to know is, is it possible to force PHP to write the uploaded contents as it receives them, or must they all be written at the end? In other words, does the file you are currently uploading have to completely fit into Memory (physical or virtual) before it can be written to disk? There is a perl script available that allows for file uploads that lets you write to disk as it's uploaded, instead of in one fell swoop at the end. Is this functionality available in PHP, and if so, where might I find it? I've never seen any mention of this ability in the documentation unfortunately.