The HttpRequestPool class provides a solution. Many thanks to those who pointed this out.
A brief tutorial can be found at: http://www.phptutorial.i
I'm pretty sure HttpRequestPool is what you're looking for.
To elaborate a little, you can use forking to achieve what you're looking for, but that seems unnecessarily complex and not very useful in a HTML context. While I haven't tested, this code should be it:
// let $requests be an array of requests to send
$pool = new HttpRequestPool();
foreach ($requests as $request) {
$pool->attach($request);
}
$pool->send();
foreach ($pool as $request) {
// do stuff
}