If you get invalid credential error when hosting your application on Heroku check your application setting:
Step 1:
Canvas Type:
Make sure IFrame is selected.
Step 2:
Open
FBUtils.phpand go to line 94, it reads
echo("<script> top.location.href='" . $authorize_url . "'</script>"); replace it by
echo("<script> window.location.href='" . $authorize_url . "'</script>"); and that’s all, you won’t get off of FB and your app will be inside the iframe!
Thys Michels Blog 
I’m having this problem too, but your instructions didn’t work for me.
I can’t actually find where it allows me to state the canvas is an iFrame in the FB developer section.
I’ve edited the FBUtils.php but am not sure if that is committed or pushed to the heroku version after I did it.
It’s not very good if their service sort of fails so easily. Surely they’d check it themselves you’d think?
Not sure about using it if it is likely to be buggy.
Hi, this does work, so confirming it for others.
My problem was making sure the changes were committed properly, and then pushed properly to Heroku.
It doesn’t really tell you if you fail in your terminal commands, and I’ve not a terminal/linux kind of guy.
Thanks for posting this though. It saved plenty of frustration.
Worked for me!
thanks 🙂