PHP Guzzle allows you to make concurrent requests, but you have to specify the "concurrency" number as a parameter.
For example:
$client = new Client();
$pool = new Pool($client, $generator($url, $data), [
'concurrency' => 5,//this lets you set how many concurrent requests can be made
'fulfilled' => function($response, $id) {
What is the maximum number of concurrent requests supported?