Facebook Registration - Check if username is available
//this show the button and, once approved from user call process.php
    <div id="fb-login-wrapper">
<div class="fb-login-button" onlogin="javascript:registerme();" size="medium"    scope="publish_stream,email">Login</div>
</div>
<div id="fb-root"></div>
<script type="text/javascript">
window.fbAsyncInit = function() {
FB.init({
appId:  '<?php echo $appId; ?>',
cookie: true,
xfbml: true,
oauth: true,
display:'popup'});
};
(function() {var e = document.createElement('script');
e.async = true;e.src = document.location.protocol +'//connect.facebook.net/en_US/all.js';
document.getElementById('fb-root').appendChild(e);}());
function registerme(){
        FB.login(function(response) {
        if (response.status === "connected")
        {
        var alldata =  {processthis: "yes"};
        $.colorbox({href:"process.php",data:alldata,onClosed:function(){ $("#fb-login-wrapper").show(); },onComplete:function(){FB.XFBML.parse(); }});
        }
    });
}
</script>


<?php

 //this is just to check what data I'm getting back, of course I will store these data on my database, but I want to check if the username is already taken.

if(isset($_SESSION['facebook_data']))
{
        //registration data sent from receive_data.php file
        $userid = $_SESSION['facebook_data']["user_id"];
        $name = $_SESSION['facebook_data']["registration"]["name"];
        $first_name = $_SESSION['facebook_data']["registration"]["first_name"];
        $last_name = $_SESSION['facebook_data']["registration"]["last_name"];
        $email = $_SESSION['facebook_data']["registration"]["email"];
        $location = $_SESSION['facebook_data']["registration"]["location"];
        $gender = $_SESSION['facebook_data']["registration"]["gender"];
        $birthday = $_SESSION['facebook_data']["registration"]["birthday"];
        $username = $_SESSION['facebook_data']["registration"]["username"];
        $password = $_SESSION['facebook_data']["registration"]["password"];

        //Now we have user data, we can store it in database or just prepare for output
        $facebookData = array('Facebook User ID'=>$userid,'Name'=>$name,'first_name'=>$first_name,'last_name'=>$last_name,'location'=>$location,'email'=>$email,'gender'=>$gender,'birthday'=>$birthday,'username'=>$username,'psw'=>$password);

        //Output this data
        echo '<pre>';
        print_r($facebookData);
        echo '<pre>';

//distroy session
session_destroy();
}
?>

<?php
require_once('config.php' );

if(isset($_POST["processthis"]) && $_POST["processthis"]=="yes")
{
$regFields='[{"name":"name"},
            {"name":"first_name"},
            {"name":"last_name"},
            {"name":"location"},
            {"name":"email"},
            {"name":"gender"},
            {"name":"birthday"},
            {"name":"username","description":"Username","type":"text"},
            {"name":"password"}]';

echo '<div id="registration_form" style="width:560px;height:330px;" align="center">
<fb:registration
fields=''.$regFields.''
redirect-uri="'.$redirect_url.'receive_data.php"
width="530">
</fb:registration>
</div>';
 }
 ?>

<?php
 require_once('config.php' );

 session_start();

    if(isset($_POST["signed_request"])){ //receive user data from facebook
    $signedresponse = parse_signed_request($_REQUEST["signed_request"], $appSect);
if($signedresponse)
{
    //Receive facebook data and redirect user to homepage with session variable
    $_SESSION['facebook_data'] = $signedresponse;
    header('Location: '.$redirect_url.'index.php');
}
     }

     //function to phrase signed data from facebook.
     function parse_signed_request($signed_request, $secret) {
      list($encoded_sig, $payload) = explode('.', $signed_request, 2);
      $sig = base64_url_decode($encoded_sig);
      $data = json_decode(base64_url_decode($payload), true);

      if (strtoupper($data['algorithm']) !== 'HMAC-SHA256') {
        die('Unknown algorithm. Expected HMAC-SHA256');
        return null;
      }
      $expected_sig = hash_hmac('sha256', $payload, $secret, $raw = true);
      if ($sig !== $expected_sig) {
        die('Bad Signed JSON signature!');
        return null;
      }
      return $data;
}

   function base64_url_decode($input) {
    return base64_decode(strtr($input, '-_', '+/'));
     }

      ?>