composer.json

{
    "require": {
        "guzzle/guzzle": "dev-master"
    }
}

guzzle.php

<pre><?php

require_once 'vendor/autoload.php';

use Guzzle\Http\Client;
use Guzzle\Plugin\Oauth\OauthPlugin;
use Guzzle\Http\Exception\ClientErrorResponseException;

/**
 * Configure the following vars by creating a private application in Xero.
 * 
 * @see http://developer.xero.com/api-overview/private-applications/
 */
$key        = 'ABCDEFGHIJKLMNOPQRSTUVWXYZABCD';
$secret     = 'ABCDEFGHIJKLMNOPQRSTUVWXYZABCD';
$privateKey = 'privatekey.pem';

$client = new Client();

// Configure the OAuth authentication
$client->addSubscriber(new OauthPlugin(array(
    'consumer_key'       => $key,
    'consumer_secret'    => $secret,
    'token'              => $key,
    'signature_method'   => 'RSA-SHA1',
    'signature_callback' => function ($baseString) use ($privateKey) {
        $signature = '';

        $privateKeyId = openssl_get_privatekey(file_get_contents($privateKey));

        openssl_sign($baseString, $signature, $privateKeyId);

        openssl_free_key($privateKeyId);

        return $signature;
    }
)));

// Configure the request to the Organisation endpoint
$request = $client->get('https://api.xero.com/api.xro/2.0/Organisation');

// Send the request to the API and handle the response
try {
    $response = $request->send();
} catch (ClientErrorResponseException $ex) {
    // Get the error response
    $response = $ex->getResponse();

    if (0 === strpos($response->getBody(), '<')) {
        throw $ex;
    } else {
        // This is an OAuth error, in url encoded string format
        $error = array();
        parse_str($response->getBody(), $error);
        throw new Exception(sprintf('%s (%s)', $error['oauth_problem_advice'], $error['oauth_problem']));
    }
}

echo htmlspecialchars($response->getBody());

?></pre>