It looks like msg_receive() allocates a memory with size $maxsize, and only then tries to receive a message from queue into allocated memory. Because my script dies with $maxsize = 1 Gib, but works with $maxsize = 10 Kib.

<?php error_reporting(E_ALL);
/**
 * Example for sending and receiving Messages via the System V Message Queue
 *
 * To try this script run it synchron/asynchron twice times. One time with ?typ=send and one time with ?typ=receive
 *
 * @author          Thomas Eimers - Mehrkanal GmbH
 *
 * This document is distributed in the hope that it will be useful, but without any warranty;
 * without even the implied warranty of merchantability or fitness for a particular purpose.
 */

header('Content-Type: text/plain; charset=ISO-8859-1');
echo "Start...\n";

// Create System V Message Queue. Integer value is the number of the Queue
$queue = msg_get_queue(100379);

// Sendoptions
$message='nachricht';     // Transfering Data
$serialize_needed=false;  // Must the transfer data be serialized ?
$block_send=false;        // Block if Message could not be send (Queue full...) (true/false)
$msgtype_send=1;          // Any Integer above 0. It signeds every Message. So you could handle multible message
                          // type in one Queue.

// Receiveoptions
$msgtype_receive=1;       // Whiche type of Message we want to receive ? (Here, the type is the same as the type we send,
                          // but if you set this to 0 you receive the next Message in the Queue with any type.
$maxsize=100;             // How long is the maximal data you like to receive.
$option_receive=MSG_IPC_NOWAIT; // If there are no messages of the wanted type in the Queue continue without wating.
                          // If is set to NULL wait for a Message.

// Send or receive 20 Messages
for ($i=0;$i<20;$i++) {
  sleep(1);
  // This one sends
  if ($_GET['typ']=='send') {
    if(msg_send($queue,$msgtype_send, $message,$serialize_needed, $block_send,$err)===true) {
      echo "Message sendet.\n";
    } else {
      var_dump($err);
    }
  // This one received
  } else {
    $queue_status=msg_stat_queue($queue);
    echo 'Messages in the queue: '.$queue_status['msg_qnum']."\n";

    // WARNUNG: nur weil vor einer Zeile Code noch Nachrichten in der Queue waren, muss das jetzt nciht mehr der Fall sein!
    if ($queue_status['msg_qnum']>0) {
      if (msg_receive($queue,$msgtype_receive ,$msgtype_erhalten,$maxsize,$daten,$serialize_needed, $option_receive, $err)===true) {
              echo "Received data".$daten."\n";
      } else {
              var_dump($err);
      }
    }
  }
}

?>

It seems that a maxsize of 2Mb is some sort of a threshold for php, above that msg_receive() starts to use a lot of CPU (with a sender that is pushing messages non-stop receiving 10000 messages jumps up from 0.01 sec to 1.5 sec on my computer) so try to stay below that thresholod if you can.

Consider this e.g. Linux situation:

<?php
//file send.php
$ip = msg_get_queue(12340);
msg_send($ip,8,"abcd",false,false,$err);
//-----------------------------------------------------
<?php
//file receive.php
$ip = msg_get_queue(12340);

msg_receive($ip,0,$msgtype,4,$data,false,null,$err);
echo "msgtype {$msgtype} data {$data}\n";

msg_receive($ip,0,$msgtype,4,$data,false,null,$err);
echo "msgtype {$msgtype} data {$data}\n";
?>

Now run: 
in terminal #1   php5 receive.php
in terminal #2   php5 receive.php
in terminal #3   php5 send.php

Showing messages from queue will flip-flop. It means you run once send.php, the message will be shown in terminal #1. Second run it will be in t#2, third #1 and so on.

This is meant to be run as your apache user in a terminal, call script in note of msg_send and they will communicate.

#! /usr/bin/env php
<?php
    $MSGKEY = 519051; // Message

    $msg_id = msg_get_queue ($MSGKEY, 0600);

    while (1) {
        if (msg_receive ($msg_id, 1, $msg_type, 16384, $msg, true, 0, $msg_error)) {
            if ($msg == 'Quit') break;
            echo "$msg\n";
        } else {
            echo "Received $msg_error fetching message\n";
            break;
        }
    }

    msg_remove_queue ($msg_id);
?>