Set user-agent in guzzle. client->setDefaultOption('headers/User-Agent', 'foo'); does not work for user-agent.

guzzle.php

<?php
require 'vendor/autoload.php';

use Guzzle\Service\Client;

// Create a client with a base URL
$client = new Client('http://requestb.in/1ivmkhd1');
$client->setUserAgent('myuseragent');
// Send a request to https://github.com/notifications
$response = $client->get('')->send();
print($response);