cURL Multi Threading with PHP
The above accepted answer is outdated, So, correct answer has to be upvoted.
http://php.net/manual/en/function.curl-multi-init.php
Now, PHP supports fetching multiple URLs at the same time.
https://github.com/krakjoe/pthreads
You may thread in PHP, the code depicted is just horrible thread programming, and I don't advise that is how you do it, but wanted to show you the overhead of 20,000 threads ... it's 18 seconds, on my current hardware which is a Intel G620 ( dual core ) with 8gigs of ram, on server hardware you can expect much faster results ... how you thread such a task is dependant on your resources, and the resources of the service you are requesting ...