Make it stop! What is proposed here is not multithreading! Just use Thread[1], or a multithreaded language. And for the love of god if you do ever use this, escape[2] your shell arguments!

[1] http://prototype.php.net/manual/en/book.pthreads.php

[2] http://prototype.php.net/manual/en/function.escapeshellarg.p...