simple-responder.php

<?php
if (!isset($_POST['msisdn'], $_POST['to'], $_POST['text'])) {
    return; // not a nexmo request
}

if ("yes" === strtolower(substr($_POST['text'], 0, 3))) {
    // here's where you need to do something to store the inbound phone number
    error_log('got message from: ' . $_POST['msisdn']);

    // reply with the message (this is a bit hacky, but should work anywhere)
    $data = [
        'api_key' => NEXMO_KEY,
        'api_secret' => NEXMO_SECRET,
        'from' => $_POST['to'], // simple way to use the same code for multiple numbers
        'to' => $_POST['msisdn'],
        'text' => "Here's a link: http://example.com"
    ];

    $url = "https://rest.nexmo.com/sms/json?" . http_build_query($data);

    $response = file_get_contents($url);

    $response = json_decode($response, true);

    if ("0" === $response['status']){
        error_log('sent reply');
    } else {
        error_log('could not send reply, status: ' . $response['status']);
    }
}