I’m writing a Facebook iframe/canvas app and would like to give users the ability to invite friends to install this app. Currently I am using Facebook’s PHP-SDK for logging in the users and all interaction with Facebook. The only things I can find about implementing the invite friends dialog either relate to FBML, which has bee deprecated; or the requests dialog: https://developers.facebook.com/docs/reference/dialogs/requests/ . The latter seems to require the use of Facebook’s Javascript SDK, which has its own methods for login.
Is the JS SDK my only option, and if so, how can I use this without forcing my users to have to login again?
Yes – currently the FB JS requests dialog is the only way. And your users won’t need to authenticate with your app – in fact they won’t need to authenticate with it all. I have a jsfiddle example with code you can look at, but all you need to specify is your app id.