Use PATH_INFO, which is part of the CGI specification and can be accessed in PHP through $SERVER['PATH_INFO']. If you have a PHP page at http://www.example.com/somepage.php, and the user hits the URL http://www.example.com/somepage.php/foo/bar, $SERVER['PATH_INFO'] contains '/foo/bar'. You can use explode('/', $_SERVER['PATH_INFO']) to split it into an array (note the 0th element will always be empty due to the leading slash).
I have heard that Apache 2 needs a config option specified to enable PATH_INFO. I recommend defining a function to get the extra arguments, so that if it comes down to it you can modify that one function to parse the whole URL if necessary.
For some reason, PATH_INFO seems to be a well-kept secret. I've heard of people doing all kinds of contortions (like mod_rewrite) to try to get equivalent functionality.
