Here is a link to a page describing one way of previewing php code in the browser:
http://tinyurl.com/dkg5e
This is as close as I can come to what you're looking for. I've never heard of a way to route php through the php parser via the file:-protocol as you (andehlu) suggest in your post. I haven't ever heard of a plugin that would let you do it either, and I have searched pretty hard for it.
[EDIT] Maybe you could try this: use terminal to send the document to php, and pipe the result to Safari? Maybe use a droplet (AppleScript or Automator perhaps) to do it for you? [/EDIT]
As for not telling you your memory is deceiving you: I've been working with php for almost eight years now (since the betas of PHP/FI 2.0), and I've never even heard of anything remotely similar to what you describe. I've consulted with a bunch of colleagues on the matter also, and I'm afraid they all come down on my side of the fence...
However, it is entirely feasible to create a plugin for Safari that will do what you want. If you really think it is worth it, you might want to try that.