First define which permissions you will need:
$par['req_perms'] = "friends_about_me,friends_education_history,friends_likes,friends_interests,friends_location,friends_religion_politics,
friends_work_history,publish_stream,friends_activities,friends_events,
friends_hometown,friends_location,user_interests,user_likes,user_events,
user_about_me,user_status,user_work_history,read_requests,read_stream,offline_access,user_religion_politics,email,user_groups";
$loginUrl = $facebook->getLoginUrl($par);
Then, check if the user has already subscribed to app:
$session = $facebook->getSession();
if ( is_null($session) ) {
// no he is not
//send him to permissions page
header( "Location: $loginUrl" );
}
else {
//yes, he is already subscribed, or subscribed just now
echo "<p>everything is ok";
// write your code here
}