What's up with these single underscores before method names?

https://github.com/facebook/facebook-php-sdk-v4/blob/master/src/Facebook/FacebookSession.php#L326

[gfosco]

These are psuedo-private methods... The underscore means you probably shouldn't be using them. Unfortunately, PHP doesn't have package protected scope, so when another class needs to access the method, we have to make it public.