Description:
------------
The multicast support in PHP 5.4 makes use of MCAST_JOIN_GROUP if it is present. 
The problem is that OSX 10.7 added the constant, but did not correctly implement 
the feature. This causes the setsockopt call to fail.

The solution to the problem is to not use MCAST_JOIN_GROUP on OSX.

Test script:
---------------
<?php
if (!extension_loaded('sockets')) {
    die('skip sockets extension not available.');
}
if (PHP_OS !== 'Darwin') {
    die('is not OSX.');
}

$socket = socket_create(AF_INET, SOCK_DGRAM, SOL_UDP);
socket_bind($socket, '0.0.0.0', 31057);

$so = socket_set_option($socket, IPPROTO_IP, MCAST_JOIN_GROUP, array(
    "group" => '224.0.0.251',
    "interface" => 0,
));
var_dump($so);


Expected result:
----------------
bool(true)

Actual result:
--------------
PHP Warning:  socket_set_option(): unable to set socket option [22]: Invalid 
argument in /Users/igor/code/react-mdns-userland/src/React/Mdns/Client.php on 
line 27
PHP Stack trace:
PHP   1. {main}() /Users/igor/code/react-mdns-userland/examples/client.php:0
PHP   2. React\Mdns\Client->query() /Users/igor/code/react-mdns-
userland/examples/client.php:10
PHP   3. socket_set_option() /Users/igor/code/react-mdns-
userland/src/React/Mdns/Client.php:27

bool(false)