I\'m trying to do exactly the same thing as in my previous Python question, but in PHP. See my previous (answered) question
PHP script from previous question does someth
If you need this other script to be run, depending on the client would not be wise.
What I'd do is use the technique described in the answer to this StackOverflow question (which points to this comment in the PHP documentation) and include the other script as your post-processing.
However, that comment was written in 2006, and things may have changed since then. Please give the technique a try (as I will be doing, just for fun) and see if it works for you :)