Ok, poor example; just consider that it's single-threaded. Pretty much the only threading support is by way of a young PECL extension called pthreads[1], which is still experimental and dangerous. Otherwise, you can use the PCTL extension[2], if you don't mind compiling PHP yourself (and if you're not on Windows, where it's not supported).

1. http://www.php.net/manual/en/intro.pthreads.php

2. http://www.php.net/manual/en/pcntl.installation.php