Currently from what I got after doing some research on the internet, there are a lot of languages that you can use for Phonegap app back-end, which are:
For PHP and Java, you can use Jquery AJAX to retrieve the data from the server-side code file. For server-side code file, you are supposed to upload them on your hosting storage.
Here is one tutorial for PHP: Link
Here is one tutorial for Firebase that I found : Link
If you ask me which data storage strategy to use, I would recommend Firebase as there are not much of a hassle. Firebase provide 1GB of storage for Realtime Database . As for PHP and Java, not many hosting company gives free hosting service. Therefore it costs some money especially when you are new to developing or a student.