FB OAuth 2.0: User has authorized app, but now app wants to request additional perms

Question

Hi,

I've implemented a sign-up page where there's a FB login button that users can click to authorize my app and allow me to fetch user details from their FB account. I'm currently also request extra perms via this login button.

E.g. <fb:login-button perms="email"></fb:login-button>

My question is, if the user refuses to grant my app access to their email, how do I find out? I know that after they authorize, I'm able to get the user details via the PHP SDK.

What should I do if i find that after $me = $facebook->api('/me');, $me['email'] does not exist? How do I request for the user to authorize again?

Thanks..