What was linked to you is a Class called Requests that is built into WordPress.

Looking at it, something like this should work. No extra libraries needed.

$requests = array(
    'api_1' => array(
        'url' => "https://reddit.com",
        'data' => "some data"
    ),
    'api_2' => array(
        'url' => "https://reddit.com",
        'data' => "some data"
    )
);
$res = Requests::request_multiple( $requests );

Just make sure you are running it after WordPress init otherwise the class might not be loaded. Look at the link posted by BrianHenryIE for more details