Reminder: Be sure to create an Facebook app before testing locally! And debugging 'signed_ request' (if user liked/disliked page) on Facebook only works live with SSL. Facebook will send some JSON details in the signed_request via the live/developer app.
I'm truly sorry for sharing my code as an image. I felt it was the easiest way to do at 2 pm. (ZzZ)
Check that algorithm is valid. Yes, I may have 'stolen' that part
Check like/disliked page