Android : Build a sync service for android / ios?

Currently I'm building an application for android/ios. This app is written in C++/QML and connects via PHP with an MySQL database-server.

A feature I really want to have build in is the ability to sync the mobile device with the database (mainly for updates from other users) while the app is not actively running. I thought the best approach to do this is to write a service that runs independent from the app. This service would be activated when you boot up your mobile device and starts syncing data when triggered by incoming push-messages from the server.

My question is if this is the best approach and if it is even possible to write such a service (with the necessary rights to run from boot) for android/ios according to their policies?

I've read a lot about this subject already, but there are a lot of contradicting answers. That's why I'm a bit lost right now.

Hope you guys could give me some advice/directions on this matter! (No spoon-feeding needed...)

Thanks upfront!