client.php

<?php declare(strict_types = 1);

function send_data($socket, $data)
{
    echo "Sending data {$data}\n";
    fwrite($socket, $data);
    $response = fread($socket, 1024);
    echo "Got response: {$response}\n";
}

$socket = stream_socket_client("tcp://127.0.0.1:47806");
send_data($socket, 'HELLO');
send_data($socket, 'WHAT YOU UP TO?');
send_data($socket, 'HAVING FUN?');

server.php

<?php declare(strict_types = 1);

$server = stream_socket_server("tcp://127.0.0.1:47806");
$serverId = (int)$server;

$readSockets = [$serverId => $server];
$writeSockets = $pendingWriteData = [];

// This defines our "protocol". It's just an array that maps "request" strings to "response" strings
$protocolDefinition = [
    'HELLO' => 'HEY THERE',
    'WHAT YOU UP TO?' => 'OH NOT MUCH',
    'HAVING FUN?' => 'YEH THIS IS GREAT!',
];
$badRequestResponse = "SORRY I DIDN'T UNDERSTAND YOU :-("; // send this back to any data we don't recognise

while (true) {
    // do anything you might need to do at the beginning of each loop tick
    echo "Loop tick\n";

    $r = $readSockets; // create copies of all watchable socket arrays
    $w = $writeSockets;
    $e = null; // we still don't care about OOB data. Very few applications do.

    // how long (max) select() will block waiting for something to happen
    $timeoutSecs = 1;
    $timeoutUSecs = 0;

    $selectResult = stream_select($r, $w, $e, $timeoutSecs, $timeoutUSecs);

    if ($selectResult === false) {
        // an E_WARNING is raised in this case, check that to find out if there's any more info about what went wrong
        echo "select() returned an error\n";
        break;
    }

    if ($selectResult > 0) {
        // if something has happened on a socket then process it

        echo count($r) . " sockets have readable activity\n";
        foreach ($r as $socket) {
            $socketId = (int)$socket;

            if ($socketId === $serverId) {
                $client = stream_socket_accept($server);
                stream_set_blocking($client, false); // set the stream to non-blocking as soon as it's created

                $clientId = (int)$client;
                $readSockets[$clientId] = $client;

                echo "Got a new client #{$clientId}\n";
            } else {
                $data = fread($socket, 1024);

                if ($data === '') {
                    unset($readSockets[$socketId]);
                    echo "Client #{$socketId} disconnected\n";
                    continue;
                }

                echo "Got data from client #{$socketId}: {$data}\n";

                $pendingWriteData[$socketId] = $protocolDefinition[$data] ?? $badRequestResponse;
                $writeSockets[$socketId] = $socket; // this is a simple example so we won't actually try to send the
                                                    // data here so that we can force the loop to detect writable
                                                    // sockets. In the real world, we'd actually try and send the data
                                                    // immediately and only queue it in the buffer if we couldn't send
                                                    // it all.

                echo "Responding with {$pendingWriteData[$socketId]}\n";
            }
        }

        echo count($w) . " sockets are writable\n";
        foreach ($w as $socket) {
            $socketId = (int)$socket;

            $data = $pendingWriteData[$socketId];
            $dataLen = strlen($data);

            echo "Sending data {$data} ({$dataLen} bytes) to client #{$socketId}\n";
            $sent = fwrite($socket, $data);

            if ($sent === false) {
                echo "Sending data failed!\n"; // E_WARNING should tell you want happened here
                unset($pendingWriteData[$socketId], $writeSockets[$socketId]);
                continue;
            }

            echo "Sent {$sent} bytes to client #{$socketId}\n";

            if ($sent === $dataLen) {
                echo "Send buffer for client #{$socketId} drained\n";
                unset($pendingWriteData[$socketId], $writeSockets[$socketId]);
                continue;
            }
            
            // if we get here, it means that we only managed to send part of the data. This won't happen unless you are
            // trying to send a lot of data (probably several KB) in one go. So here we just remove the data that we sent
            // from the start of our buffer, and leave the socket to be watched for writes so we can try again.
            $pendingWriteData[$socketId] = substr($pendingWriteData[$socketId], $sent);
        }
    }

    // do anything you might need to do at the end of each loop tick
}