In his latest blog post, Jacob Santos takes a look at a common request PHP developers hit after a while - is it possible to multitask processes/actions in php?
The ability to run two or more commands in parallel is totally sexy, but the Web is a different playing field and it doesn't make complete sense.
PHP loses control, once the output is sent and the connection is closed. Multithreading would probably make sense in streaming, but PHP already handles that for you where it makes sense for the function. Furthermore, mulithreading has its purpose in applications that continue to run and not something that closes and cleans up everything after the last command is issued.
He continues talking more about what multitasking is, a more real life example, the goods, the bads, and some of the issues that can come along with it. Finally, he suggests a feature to be added to the core of PHP - an "internal multitasking mechanism" to help aid these kinds of processes.