[Executing and parsing javascript server-side]

I have an odd problem - I need to run and parse the output of a javascript script without the user ever seeing it.  I am trying to pull data out of a script that uses data from a seperate domain that I have no control of, so I need to be able to simply strip out the information I need.

 

The step I can't figure out is getting the output.  I know that javascript is intended to run on the client and PHP shouldn't know the difference, but I need some way for PHP to run it *itself* before sending anything to the client.  Does anyone have any ideas?

 

Thanks