library-responder.php

<?php
require_once __DIR__ . '/vendor/autoload.php';

$client = new \Nexmo\Client(new \Nexmo\Client\Credentials\Basic(NEXMO_KEY, NEXMO_SECRET));

try {
    $inbound = \Nexmo\Message\InboundMessage::createFromGlobals();
} catch (Exception $e) {
    error_log('could not parse inbound message');
}

// here's where you need to do something to store the inbound phone number
error_log('got message from: ' . $inbound->getFrom());

if ('yes' === strtolower(substr($inbound->getBody(), 0, 3))) {
    $reply = $inbound->createReply("Here's a link: http://example.com");
}

// perhaps other keywords


if (!isset($reply)) {
    return;
}

try {
    $client->message()->send($reply);
} catch (Exception $e) {
    error_log('could not send reply: ' . $e->getMessage());
}