Error while using REST api in magento

I have set up magento locally in my system using XAMPP

I have created a file in root directory named dm.php with the contents

<?php
/**
* Example of products list retrieve using Customer account via Magento REST API. OAuth authorization is used
*/
$callbackUrl = "http://localhost/dm.php";
$temporaryCredentialsRequestUrl = "http://localhost/mage2/oauth/initiate?oauth_callback=" . urlencode($callbackUrl);
$adminAuthorizationUrl = 'http://localhost/mage2/oauth/authorize';
$accessTokenRequestUrl = 'http://localhost/mage2/oauth/token';
$apiUrl = 'http://localhost/mage2/api/rest';
$consumerKey = 'enhksf7u33p3snubewb6zcq0z9c63bvv';
$consumerSecret = 'p7e835cdcxofokeep749jgzz4l1e306p';

session_start();
if (!isset($_GET['oauth_token']) && isset($_SESSION['state']) && $_SESSION['state'] == 1) {
    $_SESSION['state'] = 0;
}
try {
    $authType = ($_SESSION['state'] == 2) ? OAUTH_AUTH_TYPE_AUTHORIZATION : OAUTH_AUTH_TYPE_URI;
    $oauthClient = new OAuth($consumerKey, $consumerSecret, OAUTH_SIG_METHOD_HMACSHA1, $authType);
    $oauthClient->enableDebug();

    if (!isset($_GET['oauth_token']) && !$_SESSION['state']) {
        $requestToken = $oauthClient->getRequestToken($temporaryCredentialsRequestUrl);
        $_SESSION['secret'] = $requestToken['oauth_token_secret'];
        $_SESSION['state'] = 1;
        header('Location: ' . $adminAuthorizationUrl . '?oauth_token=' . $requestToken['oauth_token']);
        exit;
    } else if ($_SESSION['state'] == 1) {
        $oauthClient->setToken($_GET['oauth_token'], $_SESSION['secret']);
        $accessToken = $oauthClient->getAccessToken($accessTokenRequestUrl);
        $_SESSION['state'] = 2;
        $_SESSION['token'] = $accessToken['oauth_token'];
        $_SESSION['secret'] = $accessToken['oauth_token_secret'];
        header('Location: ' . $callbackUrl);
        exit;
    } else {
        $oauthClient->setToken($_SESSION['token'], $_SESSION['secret']);
        $resourceUrl = "$apiUrl/products";
        $oauthClient->fetch($resourceUrl);
        $productsList = json_decode($oauthClient->getLastResponse());
        print_r($productsList);
    }
} catch (OAuthException $e) {
    print_r($e);
}

But this is giving me the following error

Fatal error: Class 'OAuth' not found in D:\Webserver\xampp\htdocs\dm.php on line 19

Can anybody shed some light on this Thanks

Since oauth is not possible to install in xampp windows i changed the contents of my dm.php file to this.

<?php
$ch = curl_init('http://localhost/mage2/api/rest/customers');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$customers = curl_exec($ch);
echo $customers;
?>

Now i am getting an error like this

{"messages":{"error":[{"code":403,"message":"Access denied"}]}}

What am i doing wrong?