Question

I’m trying to promote my app using app center but from the Web preview I can’t visit the web site.
The link that is called is http://www.myappsite.it/?fb_source=appcenter&fb_appcenter=1&code=a_long_string

From the index.php of myappsite I use this peace of php code to get the user coming from facebook

$code = $_REQUEST["code"];

if($_REQUEST['state'] == $_SESSION['state'] && !$user && strlen($code) > 0) 
{
$token_url = "https://graph.facebook.com/oauth/access_token?" . 
"client_id=" . $appId . "&redirect_uri=" . urlencode("http://www.myappsite.it/") . 
"&client_secret=" . $secret . "&code=" . $code;

$response = @file_get_contents($token_url);
$params = null;

parse_str($response, $params);

$graph_url = "https://graph.facebook.com/me?access_token=" .     $params['access_token'];

$user = json_decode(file_get_contents($graph_url));
echo("Hello " . $user->name);
}

but $params[‘access_token’] is empty because the $token_url returns

{
"error": {
    "message": "Error validating verification code.",
    "type": "OAuthException",
    "code": 100
    }
}

how can I get the user logged in from tha app center preview web page?

Answer 1

From the Authenticated Referrals part of the docs (the App centre counts as an authenticated referral)

See A special consideration when using the Query String setting:

If you would like to use the server-side authentication flow it is
important to make sure you are passing the redirect_uri parameter
correctly when exchanging your code for an access token. You should
set your redirect_uri parameter to the click-through URL to your site
without the code parameter. In most cases the URL will look something
like:

http://www.example.com/?fb_source=search&code=CODE_HERE Once you
strip the code parameter it will become:

http://www.example.com/?fb_source=search which is the value that
should be set in redirect_uri. Please make sure that this logic is
dynamic as the query parameters appended to your click-through URL may
be subject to change.

Using your example from above, this means your redirect_uri parameter should be http://www.myappsite.it/?fb_source=appcenter&fb_appcenter=1