msg_receive

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);
?>