Using Zend_OAuth_Consumer to authorize with Flickr API
===

For those willing to use the Zend_OAuth_Consumer solution, which I highly recommend, as it handles all the trivial steps needed to process an OAuth request, and all the difficult procedures that follow the response, here is my working code:

1) Establish the consumer instance:
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    $consumer = new Zend_Oauth_Consumer(array(
        'callbackUrl' => FLICKR_REDIRECT_URL /* from my app settings */,
        'siteUrl' => 'http://www.flickr.com/services/oauth',
        'consumerKey' => FLICKR_API_KEY,
        'consumerSecret' => FLICKR_API_SECRET,
        'requestTokenUrl' => FLICKR_REQUEST_TOKEN_URL /* www.flickr.com/services/oauth/request_token */,
        'accessTokenUrl' => FLICKR_ACCESS_TOKEN_URL /* www.flickr.com/services/oauth/access_token */,
        'authorizeUrl' => FLICKR_AUTHORIZE_URL /* www.flickr.com/services/oauth/authorize */
    ));

2) Get the request token and store it in a $_SESSION variable (or use whatever other storage mechanism you want). Be sure to serialize the return object as a string for the session variable:

    $token = $consumer->getRequestToken();

    $_SESSION['FLICKR_REQUEST_TOKEN'] = serialize($token);

3) Where I had issues setting the permissions, use the consumer instance's method to redirect, and set your perms at this point (i.e. 'read', 'write', 'delete', etc...):

    $consumer->redirect(array("perms" => FLICKR_API_PERMS /* 'read', 'write', 'delete', etc... */));

4) On the page that you redirected to (i.e. FLICKR_REDIRECT_URL), create a new consumer instance, and now grab an access token, store it in the $_SESSION, and then unset the session request token variable:

    $consumer = new Zend_Oauth_Consumer(array(
        'callbackUrl' => FLICKR_REDIRECT_URL /* from my app settings */,
        'siteUrl' => 'http://www.flickr.com/services/oauth',
        'consumerKey' => FLICKR_API_KEY,
        'consumerSecret' => FLICKR_API_SECRET,
        'requestTokenUrl' => FLICKR_REQUEST_TOKEN_URL /* www.flickr.com/services/oauth/request_token */,
        'accessTokenUrl' => FLICKR_ACCESS_TOKEN_URL /* www.flickr.com/services/oauth/access_token */,
        'authorizeUrl' => FLICKR_AUTHORIZE_URL /* www.flickr.com/services/oauth/authorize */
    ));

    $token = $consumer->getAccessToken($_GET, unserialize($_SESSION['FLICKR_REQUEST_TOKEN']));

    $_SESSION['FLICKR_ACCESS_TOKEN'] = serialize($token);

    $_SESSION['FLICKR_REQUEST_TOKEN'] = null;

5) At this point you should be able to make authorized requests to Flickr:

    $token = unserialize($_SESSION['FLICKR_ACCESS_TOKEN']);
    $client = $token->getHttpClient(array(
        'callbackUrl' => FLICKR_REDIRECT_URL /* from my app settings */,
        'siteUrl' => 'http://www.flickr.com/services/oauth',
        'consumerKey' => FLICKR_API_KEY,
        'consumerSecret' => FLICKR_API_SECRET,
        'requestTokenUrl' => FLICKR_REQUEST_TOKEN_URL /* www.flickr.com/services/oauth/request_token */,
        'accessTokenUrl' => FLICKR_ACCESS_TOKEN_URL /* www.flickr.com/services/oauth/access_token */,
        'authorizeUrl' => FLICKR_AUTHORIZE_URL /* www.flickr.com/services/oauth/authorize */
    ));
    $client->setUri("http://api.flickr.com/services/rest/");
    $client->setMethod(Zend_Http_Client::GET);
    $client->setParameterGet("method", "flickr.contacts.getList");
    $client->setParameterGet("api_key", FLICKR_API_KEY);
    $response = $client->request();

    header('Content-Type: text/xml');
    echo $response->getBody();

I hope this helps someone, greatly. Code karma =)