This is a question of opinion, so I’m not sure this is the best forum. However you have several options, including some that could be completely free.
Rails and PHP as you have mentioned can be used to create a backend using mysql as the data layer. If you are new to both of those languages, you might look at Python using one of the many frameworks for it. If your app is completely iOS based, you might also look into using CloudKit, which is free up to certain sizes (which it sounds like you could easily stay below). The advantage of CloudKit would be that you don’t even have to host the service on your own hardware. There are a few other similar options as well, included Firebase and Parse which both have free tiers that likely would provide all the storage you need.
With any of these three, you’d be using the API in swift in your iOS project, and not having to learn a new language.
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solved What type of backend to use for iOS app [closed]