Via the Zend Developer Zone, I found out about this new article from Rob Young about using PHP-MIO to create non-blocking I/O.

A couple of weeks ago I was thinking about non-blocking I/O in PHP, specifically about how clunky PHP's select implementation is. I say clunky because it's not bad, it's just not as easy to use as it could be.

[...] I have to remember which streams I'm interested in writing to, which streams I'm interested in reading from and when I get to accepting connections, which streams are server sockets that I'm interested in accepting connections on. I'm lazy, I don't want to have to do that , I want a library to handle all that for me. At this point I decided to implement something similar to Java's non-blocking I/O in PHP5. This is now finished and up on sourceforce (under the name of phpmio). In this article I hope to give you enough information to get up and running with the package.

He explains what multiplexed I/O is (including code), what PHP-MIO is (yes, including code), and a more real life example of how to use it - a simple server that accepts connections and spits back a simple response.