On DZone.com there's a recent tutorial about combining PHP and Flash to make a simple file upload tool for your site.

User contributed content; it is core to the web. [...] This article will cover how to upload content to PHP, first from an HTML form, and then from a Flash-based form. We will explore the challenges you will encounter, and propose solutions with an emphasis on the client.

They show the normal file upload method for using PHP to handle the incoming content (explaining each piece along the way) and then include the Flash code you'll need to make things work similarly. They include some checking on things like file extension, file size, file type filtering and a progress indicator. They also show you how to give it multiple file capability. You can download the code in a complete zip package.