Don't allow other pages to install my tab app.

gistfile1.php

<?php

$app_secret = 'APPSECRET';

$signed_request = parse_signed_request($_REQUEST['signed_request'], $app_secret);

$page_whitelist = array(PAGEID1, PAGEID2);

if (in_array($signed_request['page']['id'], $page_whitelist)) {
    // do stuff
} else {
   // output some error message
}