php - How can I make a facebook app work only on my page?

Solution:

This is somewhat crude but should work. This will check if the current page ID is your page ID and if not display an error message.

<?php
    // Include the Facebook PHP SDK
    require('includes/fbSDK/facebook.php');

    // Enter your app details
    $app_id = "xxxxxxxxxxx";
    $app_secret = "xxxxxxxxxxxxx";
    $facebook = new Facebook(array(
        'appId' => $app_id,
        'secret' => $app_secret,
        'cookie' => true
    ));

    $signed_request = $facebook->getSignedRequest();

    // Get the curren page ID
    $page_id = $signed_request["page"]["id"];

    // Enter your page ID
    $mypageID = '123456789';

    if ($page_id == $mypageID):
        include('/your/app/files.php');
    elseif ($page_id != $mypageID):
        echo "This app only works on a single page!";
    endif;
?>
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Here are some examples of what you can check for:

$page_id = $signed_request["page"]["id"];
$page_admin = $signed_request["page"]["admin"];
$like_status = $signed_request["page"]["liked"];
$country = $signed_request["user"]["country"];
$locale = $signed_request["user"]["locale"];
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You can for example use this method to require someone to like a page before you show content.

Hope this helps!