Content Negotiation, Caching and PHP

Hello!

I have quite a technical question and I was hoping one of you Apache
Techy guys or gals could help me...

Let me give you a quick summary:

I have a PHP script that connects to a database, grabs some data and
then creates an XML file. This XML file is then passed through to a XSL
stylesheet. The XSL stylesheet that is used is entirely determined on
the Accept-Type header from the client. So, if the user-agent can
accept XHTML, based on the application/XHTML+XML mime, then the XSL
stylesheet myfunction.xhtml.xsl is loaded. If the user-agent prefers
HTML, then the XSL stylesheet myfunction.html.xsl is loaded. You get
the picture.

The result of this PHP script is currently just echoed out. The correct
response mime is set and everything is great.

However, I'm exploring was in which this PHP script can write the
content of the returned document as a file, so that Apaches Content
Negotiation can grab the static file rather than execute the PHP script
each time.

This is pretty straight forward in the PHP script: when the script is
called, the content is written into the same directory, and the file
myfunction.xhtml is created (or whatever mime type was requested).

Now, imagine if you will, this script has been called for pretty much
every mime type the function supports, so the directory is full of
files, all initially named the same, but the extention is set for
compatability with Apache's content negotiation.

So, a client requests the URL mysite.com/myfunction, and Apache's
content negotiation kicks in and returns the appropriate static file to
the client. Awesome.

Problem: the data in the database changes, meaning I need the php file
to execute again to create the appropriate files.

Ideally I'd like to be able to specify that all the myfunction
generated static files be "deleted" after, say, an hour since creation,
thus forcing Apache to select the PHP file and execute it, resulting in
a document being returned to the client, AND a copy being created and
stored in the same directoy. Any additional requests would then be
caught by Apache's content negotiation and the static file served.

I say "deleted" in quotes, as it is not necessarily required that they
be phsyically deleted as the PHP script could simply overwrite any
existing files.

Is it possible that if I delete a static file, can I use content
negotiation to default to the PHP file if a file cant be matched (by
say, setting the Q value to 0.1 for myfunction.php) and deleting the
myfunction.xhtml file?

Does anyone understand what I'm babbling about or should I just call
the PHP file directly each time, and take the overhead of PHP for each
request on the chin?

Any thoughts or musings would be appreciated!

Cheers,

Ben