Facebook Invite takes me to facebook homepage

Question

i am trying to add facebook invite feature on my website.

when i load the invitte page, it redirect to facebook, if i key in my password, it takes me to my personal facebook homepage (and does nothing more)

This is my code

<?php
//?fb_force_mode=fbml
require_once 'facebook.php';
$api_key = 'xxx';
$secret = 'xxx';

// Names and images 
$app_name = "xxx"; 
$app_url = "xxx"; 

// Assumes application is at http://apps.facebook.com/app-url/ 
$app_image = "Application image URL"; 
$invite_href = "invite.php?fb_force_mode=fbml"; 

// Rename this as needed require_once 'facebook.php'; 
$facebook = new Facebook($api_key, $secret); 
$user = $facebook->require_login(); 

if(isset($_POST["ids"])) { 
  echo "<center>Thank you for inviting ".sizeof($_POST["ids"])." of your friends on <b><a href=\"http://apps.facebook.com/".$app_url."/\"&gt;".$app_name."&lt;/a&gt;&lt;/b&gt;.&lt;br&gt;&lt;br&gt;\n"; 
  echo "<h2><a href=\"http://apps.facebook.com/".$app_url."/\"&gt;Click here to return to ".$app_name."</a>.</h2></center>"; 
} else { 
  // Retrieve array of friends who've already added the app. 
  $fql = 'SELECT uid FROM user WHERE uid IN (SELECT uid2 FROM friend WHERE uid1='.$user.') AND is_app_user = 1'; 
  $_friends = $facebook->api_client->fql_query($fql); 

  // Extract the user ID's returned in the FQL request into a new array. 
  $friends = array(); 

  if (is_array($_friends) && count($_friends)) { 
    foreach ($_friends as $friend) { $friends[] = $friend['uid']; } } 

  // Convert the array of friends into a comma-delimeted string. 
  $friends = implode(',', $friends); 

  // Prepare the invitation text that all invited users will receive. 
  $content = "<fb:name uid=\"".$user."\" firstnameonly=\"true\" shownetwork=\"false\"/> has started using <a href=\"http://apps.facebook.com/".$app_url."/\"&gt;".$app_name."&lt;/a&gt; and thought it's <u>so cool even you should try it out</u>!\n". 
             "<fb:req-choice url=\"".$facebook->get_add_url()."\" label=\"Add ".$app_name." to your profile\"/>"; 
?> 
  <fb:request-form action="<? echo $invite_href; ?>" method="post" type="<? echo $app_name; ?>" content="<? echo htmlentities($content); ?>" image="<? echo $app_image; ?>"> 
    <fb:multi-friend-selector actiontext="Here are your friends who don't have <? echo $app_name; ?> yet. Invite whoever you want -it's free!" exclude_ids="<? echo $friends; ?>" /> 
  </fb:request-form> 
<? 
} 
?>

How can i fix it thanks