On DevShed today there's a new tutorial showing how to build file upload functionality into your scripts.

If you're a PHP developer who has built a certain number of web applications, then it's quite probable that you've already worked with HTTP file uploads. [...] First I'm going to teach you how to handle file uploads using a procedural approach, and then, with the topic well underway, by way of the object-oriented paradigm.

The introduce the beginners out there to the $_FILES array (a superglobal) that contains the details about the file(s) that have been submitted. Next comes the construction of a simple form and how to handle the submission on the PHP side.