Anyway, I can't offer insight on what factors went in to deciding what SDKs to build and support officially, but I'll pass this along as a feature request for an official PHP SDK. I can't promise if or when that might be implemented though.
We love to learn from the educators who use Dropbox. Whether you teach kids, teens, adults or a combination of all three, we want to know what apps and integrations you use with Dropbox to help with teaching. Which of the ones below is your favorite, or most used tool?