I'm placing a bid, mainly to give you some honest feedback. Unfortunately, PHP cannot run multiple concurrent threads. It can only process one request and deliver one response. Any functional programming elements must be run synchronously.
However, there are ways to get the benefits of a multithreaded language and still use PHP. It turns out that PHP is exceptionally good at calling external web services using cURL. So, if your application needs to fire off a series of parallel processes, you can write it in a language that supports it, and expose it as a web service to your PHP project.
I have recently completed a similar process, where I created a RESTful Python service to run on a different port only accessible to the local machine (for security). It's job was to spin up the multiple concurrent API calls, and then return the consolidated value back as a JSON object. The PHP script would then call the local python service to get the data. It cut the total processing time from over a minute to about 6 seconds. This allowed me to use PHP for all of my external security and routing, and Python for the multithreading.
I think you can achieve the outcome you are looking for, but not in PHP directly. It is hard to know exactly the impact, but I would be happy to give you more information if you can provide additional information on your project. Thank you!