尽管在API Explorer中工作，FQL查询仍会返回“无效的API密钥”

I'm trying the following query:

SELECT uid FROM user WHERE is_app_user AND uid IN (SELECT uid2 FROM friend WHERE uid1 = me()) ORDER BY rand()

And it seems to work using the Graph API Explorer tool. However when I try doing it from my PHP code, I keep getting the error Invalid API Key, despite having every single permission possible enabled for it.

Anything I might be missing here?

So it turns out some tables require that you specify your application's private key.