Secure AuthSub using the Zend PHP library 1.6+

After uploading a public certificate to https://www.google.com/accounts/ManageDomains, here's how to use the Zend PHP 1.6+ library to work with secure AuthSub. This example uses the Google Health Data API

<?
function setupClient($singleUseToken = null) { 
  $client = null;  

  // Fetch a new AuthSub token?
  if (!$singleUseToken) {
    $next = 'http://' . $_SERVER['HTTP_HOST'] . $_SERVER['PHP_SELF'];
    $scope = 'https://www.google.com/health/feeds';
    $authSubHandler = 'https://www.google.com/health/authsub';    
    $secure = 1;
    $session = 1;
    $permission = 1;  // 1 - allows posting notices && allows reading profile data
    $authSubURL =  Zend_Gdata_AuthSub::getAuthSubTokenUri($next, $scope, $secure, $session, $authSubHandler);
    
    $authSubURL .= '&permission=' . $permission;
    
    echo '<a href="' . $authSubURL . '">Link your Google Health Account</a>';
  } else {
    $client = new Zend_Gdata_HttpClient();
    
    // This sets your private key to be used to sign subsequent requests
    $client->setAuthSubPrivateKeyFile('/path/to/myrsakey.pem', null, true);

    $sessionToken = Zend_Gdata_AuthSub::getAuthSubSessionToken(trim($singleUseToken), $client);
    // Set the long-lived session token for subsequent requests
    $client->setAuthSubToken($sessionToken);
  }
  return $client;
}
?>

Use this function like this:

$client = setupClient(@$_GET['token']);
if ($client) {
  // Query a feed
} else {
  exit(); // Just display the AuthSub link
}

Update:Read the new documentation on using AuthSub for PHP as well as all the other client libraries.