The only way to get the users e-mail address is to request extended permissions
on the email field. The user must allow you to see this and you cannot get the
e-mail addresses of the user's friends.

http://developers.facebook.com/docs/authentication/permissions

You can do this if you are using Facebook connect by passing scope=email in the 
get string of your call to the Auth Dialog.

I'd recommend using an SDK (https://github.com/facebook/facebook-php-sdk)
instead of file_get_contents as it makes it far easier to perform the Oauth 
authentication (http://developers.facebook.com/docs/authentication/).