title: Persistent flag that user is on facebook?
body: <p>I'm in the middle of designing a mobile site for our main ecommerce site. Because the site is composed of inflexible legacy code I've opted to look up the users user agent string and identify them as a mobile user each page request. That way no changes to the url structure are needed. This seems to be working nicely so far.</p>
  
  <p>However, I thought it may be kind of cool to use this mobile version so that users can browse our ecommerce site on facebook via iframe (the dimensions are perfect). But, unlike the mobile browsers, I am having trouble finding a persistent way to identify the user as a facebook user. I know facebook sends a $_POST variable the first time a page is viewed via iframe, and I could simply just store that in a session variable and be done with it. The issue that arises though is that what if the user visits with facebook, gets marked as a facebook user in their session, then visits our regular ecommerce site? Well, they'd still be identified as a facebook user and get served the facebook version, which is not ideal.</p>