If doing it on your webapp, don't! You're doing it wrong. Otherwise why would you need php specifically? I mean I guess you could, but what advantage does it poses comparing to other languages available in pretty much every *nix system nowadays (perl, python)?
For the record, I have a nice collection of downvotes for defending PHP on point-and-laugh-at-PHP threads here at HN. I just don't see how the use case you're presnting is realistic.