Current rough outline of AsyncThread's API. (work in progress) If it works as well as I hope it does, expect a repo with an extension implementing it.

gistfile1.txt

<?php
if (empty($_SESSION['threadName'])) {
  $t = new AsyncThread("util/handler.php");

  // By setting a name, it becomes persistent and may be resumed later
  $t->setName($_SESSION['threadName'] = uniqid());
} else {

  // Previously started AsyncThread instance with still-running script
  $t = AsyncThread::Resume($_SESSION['threadName']);
}
$t->recvMessageSetBlocking(true);
$t->recvMessageSetPeek(false);

// Thread $t is running concurrently with main request
// Messages can be pushed in with sendMessage($code, $message = null)
$t->sendMessage(ADD_NUMBERS, array(1, 2));

// And popped out with one of:
//   recvMessage()
//   recvMessageRange($minCode, $maxCode)
//   recvMessageCodes(Array $codes)
list($code, $message) = $t->recvMessage();
if ($code == TOTAL) {
  echo "The other thread says our total is $message\n";
}
?>

In util/handler.php (the script we setup to run in an async thread:
<?php
$t = AsyncThread::Self();
$t->recvMessageSetBlocking(true);
$t->recvMessageSetPeek(false);

// Messages send by the request process are received inside the async thread
// The same way the request thread receives messages from us:
for(;;) {
  list($code, $message) = $t->recvMessage();
  switch($code) {
    case ADD_NUMBERS:
      $result = array_sum($message);
      $t->sendMessage(TOTAL, $result);
      break;
  }
}
?>


ADD_NUMBERS and TOTALS are app defined message codes such as:
<?php
define('ADD_NUMBERS', 1);
define('TOTAL', 2);
?>


In practice, you'd probably subclass AsyncThread to provide helper functions for sending and receiving the messages in a way that makes sense to your app.  Probably building your handler loop into that class as well.