phpFlickr authentication problems

I am trying to authenticate with phpFlickr API but I keep getting the
error: Oops! The API key or signature is invalid. I am using the
correct keys but I am having this problem with two different accounts.
There must be something that's missing. I copied the example used in
the phpFlickr site and used it in the same way:

//include this file for Flickr
require_once("../includes/phpFlickr.php");

//create a flickr instance
$f = new phpFlickr("[API_KEY]", "[SHARED_SECRET]");

//Authenticates the user for Flickr
$f->auth();
$token = $f->auth_checkToken();

// Find the NSID of the username inputted via the form
$nsid = $token['user']['nsid'];

// Get the friendly URL of the user's photos
$photos_url = $f->urls_getUserPhotos($nsid);

// Get the user's first 36 public photos
$photos = $f->photos_search(array("user_id" => $nsid, "per_page" => 36));

// Loop through the photos and output the html
foreach ($photos['photo'] as $photo)
{
echo "<a href=$photos_url$photo[id]>";
echo "<img border='0' alt='$photo[title]' ".
"src=" . $f->buildPhotoURL($photo, "Square") . ">";
echo "</a>";
$i++;

// If it reaches the sixth photo, insert a line break
if ($i % 6 == 0)
{
echo "<br>\n";
}
}




 
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