facebook 'req_perms' and 'ext_perms ' what's the difference?

I'm using the facebook php sdk 2.1.2 in my web app. I use the getLoginUrl method to request permission t开发者_C百科o the user who's logging in my site.

$facebook->getLoginUrl(array( 
'req_perms'=> 'email,read_stream,user_birthday,user_hometown,user_photos'));

I was wondering if there's a way User can ask for optional permission.And what's the difference between req_perms and ext_perms?

best regards

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req_perms are permissions that you can set as being 'required' by your application before they can begin using an application.

ext_perms are optional permissions that you can request to enhance the experience for users, the user has the option to specifically allow/disallow each of these permissions on an individual basis.

edit: req_perms and ext_perms have been depreciated and are replaced by a the single 'scope' parameter (which the values will determine what permissions are optional) - thanks Igy

ie: https://developers.facebook.com/docs/authentication/permissions/#extended_perms

If you are using the Enhanced Auth Dialog, extended permissions are presented on the second page of the dialog and are individually user-revocable. If you request these permissions, the user can choose to not grant one or more of them. Your application should not expect the user to always grant these permissions if requested.