Simple 0MQ client/server pattern

client.php

<?php
// Client: Pings server, gets reply. Exits.

$zmq = new ZMQContext;
$sock = new ZMQSocket( $zmq, ZMQ::SOCKET_REQ );
$sock->connect('tcp://localhost:5555');

echo "Pinging..\n";
$sock->send('Ping');

echo "Client waiting..\n";
$pong = $sock->recv();
echo $pong," in reply. exiting.\n";

server.php

<?php
// Server: Waits for ping, sends reply. Repeats.

$zmq = new ZMQContext;
$sock = new ZMQSocket( $zmq, ZMQ::SOCKET_REP );
$sock->bind('tcp://*:5555');

while( true ){
    echo "Server waiting..\n";
    $ping = $sock->recv();
    echo "Acknowledging ",$ping,"..\n";
    $sock->send('Pong');
}

Run server.php in one terminal window.
Open a second terminal and run client.php

The client sends a ping, receives a reply and then exits. This is all done synchronously so each end of the socket hangs on the recv call.