What's the best way to create a single-file upload form using PHP?

I've found a few samples online but I'd like to get feedback from people who use PHP daily as to potential security or performance considerations and their solutions.

Note that I am only interested in uploading a single file at a time.

Ideally no browser plugin would be required (Flash/Java), although it would be interesting to know the benefits of using a plugin.

I would like to know both the best HTML form code and PHP processing code.