500 Error After Login Approval PHP Example

[sdooleyFJUHSD]

I'm using your php example straight out, with the only changes being the key, secret, and callback url (back to itself).

The login button shows and works, the Khan Academy authorization shows and works, when you click "accept" I get the 500 error. The URL of the error shows:
oauth_token_secret=####&oauth_verifier=######&oauth_token=#######

This seems correct to me, so where is it hanging up? What am I missing?