File extension for PHP's serialization format?

I have a web application (not written in PHP) that can return data in various formats, including JSON and PHP's serialization format. At the moment my URL scheme looks like this:

    staff/engineering?format=json

but I'd like to switch to using a file extension to denote the format:

    staff/engineering.json

But what file extension should I use for PHP's serialization format? Obviously it can't be .php - aside from being inaccurate (it's not PHP code), using this extension would probably trigger the web server into trying to run a (nonexistent) PHP script. Thanks, Hamish Lawson