Wez Furlong - Guru - Multiplexing:

"People often assume that you need to fork or spawn threads whenever you need to do several things at the same time - and when they realize that PHP doesn't support threading they move on to something less nice, like perl.

The good news is that in the majority of cases you don't need to fork or thread at all, and that you will often get much better performance for not forking/threading in the first place.

[…] You can use stream_select() to wait on (almost!) any kind of stream - you can wait for keyboard input from the terminal by including STDIN in your read array for example, and you can also wait for data from pipes created by the proc_open() function."