title: Referrer URL from facebook form POST
body: <p>I'm posting a form from a Facebook application to a form processor on my web server and then redirecting to a thank you page.</p>
  
  <p>I want to ensure nothing else can post to this form so was looking to restrict it to only receive input from the Facebook app. The problem I'm coming up against is when I try to find the <code>$_SERVER['HTTP_REFERER']</code> it isn't passed from Facebook. The value isn't even null, it's not even shown.</p>
  
  <p>Is there anything else I could possibly use to lock down this form processor?</p>
  
  <p>Cheers</p>
  
  <p>Tom</p>