phpMQTT non-blocking publish & recieve

Question

I am writing a program connecting a web service in PHP to an MQTT broker. The broker is running Mosquitto on a Raspberry Pi.

The idea is to have the web service send a request (a form is submitted) and then send a publish to the MQTT broker and then wait for a reply.

However, the problem is that the loop seems to cause a PHP fatal error due to the fact that it is an endless loop.

I've tried adding the quitstop() function to quit the loop after a message is received, but the program crashes before it reaches that point.

MQTT is still very new to me, but I need to send the request and then keep the loop open until I receive the answer in order to proceed with my program.

This is the code to handle the form submit:

require("phpMQTT.php");

$server = "xxx.xxx.xxx.xx";              // change if necessary
$port = 1883;                       // change if necessary
$username = "username";             // set your username
$password = "password";             // set your password
$client_id = "phpMQTT-request-1234";     // make sure this is unique for connecting to sever - you could use uniqid()

$mqtt = new phpMQTT($server, $port, $client_id);

$msg = $_POST['box'];
if (!empty($msg)) {
    if ($mqtt->connect(true, null, $username, $password)) {
        $mqtt->publish("dev/test", $msg, 0);
        $mqtt->close();
    }
    subscribeToTopic($mqtt);
}

function subscribeToTopic($mqtt)
{
    $topics['dev/test'] = array("qos" => 0, "function" => "procmsg");
    $mqtt->subscribe($topics, 0);
    while ($mqtt->proc()) {

    }
    $mqtt->close();

}
function procmsg($topic, $msg)
{
    global $mqtt;
    echo $msg;
    quitstop($mqtt);
}

function quitstop($mqtt)
{
    $mqtt->close();
}