Examples of how to use Magento OAuth that actually work!

admin_add_product_example.php

<?php

$callbackUrl = 'http://'.$_SERVER['HTTP_HOST'].$_SERVER['PHP_SELF']; // This scripts url
$magentoUrl = 'http://example.com/'; // The url of your magento installation
$temporaryCredentialsRequestUrl = $magentoUrl . 'oauth/initiate?oauth_callback=' . urlencode($callbackUrl);
$adminAuthorizationUrl = $magentoUrl . 'admin/oauth_authorize';
$accessTokenRequestUrl = $magentoUrl . 'oauth/token';
$apiUrl = $magentoUrl . 'api/rest';
$consumerKey = 'OAUTH CONSUMER KEY'; // Get from System > Web Services > REST - OAUTH Consumers > Edit Consumer > Key
$consumerSecret = 'OAUTH CONSUMER SECRET'; // Get from System > Web Services > REST - OAUTH Consumers > Edit Consumer > Secret

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";
        $productData = json_encode(
            array(
                'type_id' => 'simple',
                'attribute_set_id' => 4,
                'sku' => 'simple' . uniqid(),
                'weight' => 1,
                'status' => 1,
                'visibility' => 4,
                'name' => 'Simple Product',
                'description' => 'Simple Description',
                'short_description' => 'Simple Short Description',
                'price' => 99.95,
                'tax_class_id' => 0
            )
        );
        $oauthClient->disableRedirects();
        $headers = array('Content-Type' => 'application/json', 'Content_Type' => 'application/json', 'Accept' => '*/*');
        $oauthClient->fetch($resourceUrl, $productData, 'POST', $headers);
        $response = json_decode($oauthClient->getLastResponse());
        print_r($response);
    }
} catch (OAuthException $e) {
    print_r($e->getMessage());
    echo "<br>";
    print_r($e->lastResponse);
}

admin_get_products_example.php

<?php

$callbackUrl = 'http://'.$_SERVER['HTTP_HOST'].$_SERVER['PHP_SELF']; // This scripts url
$magentoUrl = 'http://example.com/'; // The url of your magento installation
$temporaryCredentialsRequestUrl = $magentoUrl . 'oauth/initiate?oauth_callback=' . urlencode($callbackUrl);
$adminAuthorizationUrl = $magentoUrl . 'admin/oauth_authorize';
$accessTokenRequestUrl = $magentoUrl . 'oauth/token';
$apiUrl = $magentoUrl . 'api/rest';
$consumerKey = 'OAUTH CONSUMER KEY'; // Get from System > Web Services > REST - OAUTH Consumers > Edit Consumer > Key
$consumerSecret = 'OAUTH CONSUMER SECRET'; // Get from System > Web Services > REST - OAUTH Consumers > Edit Consumer > Secret

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->disableRedirects();
        $headers = array('Content-Type' => 'application/json', 'Content_Type' => 'application/json', 'Accept' => '*/*');
        $oauthClient->fetch($resourceUrl, array(), 'GET', $headers);
        $response = json_decode($oauthClient->getLastResponse());
        print_r($response);
    }
} catch (OAuthException $e) {
    print_r($e->getMessage());
    echo "<br>";
    print_r($e->lastResponse);
}

I am getting the following error while

@HemanYadav you are missing the php OAuth extension.

...
session_start();
if (!isset($_SESSION['state'])) $_SESSION['state'] = null; // FIX: PHP Notice:  Undefined index: state in ...

...