In a recent post to his blog David Müller has taken a look at parallel processing in PHP using a few different methods - system calls, fork, and curl.

Since PHP does not offer native threads, we have to get creative to do parallel processing. I will introduce 3 fundamentally different concepts to emulate multithreading as good as possible.

For each of the technologies mentioned above, he provides a simple bit of sample code that does simple tasks like echoing out strings and writing to files. He also includes some benchmarks (take them with a grain of salt) of the three different methods showing how many iterations they could run through in ten seconds. He includes the benchmarking script if you'd like to try it out yourself.