The idea of parallel processing is when you take an atomic transaction or operation called a 'process' and run a couple of those at the same time. [...] In this example I had a PHP CLI script. The purpose of this script was to process remote documents and save it local. It fetched the document from a remote location, processed it and saved the result local - let's call this the transaction. Transactions were done sequentially. It took about 1 second for a transaction to complete and the script had to do roughly 3500 transactions.

Obviously, this script/testing method was not very effective, so he went searching for an alternative. The best option came in the shape of forking processes - spawning off a separate script (via PHP's forking functionality) to do the work on one or multiple entries. It uses the pcntl_fork and pcntl_waitpid functions to spawn and manage these children. Once they're all finished - working in parallel - they return back to the main script to wait for the slower ones to catch up.