I'm an experienced web developer in PHP, HTML, Javascript, MySQL, CSS and I'm running communities where people can register to be able to login and do some stuff.

Now more and more people are requestion an App, I told them that I have no time and experience to develope Apps with many functions for such complex communities I am running, but then the users told me what would be enough for them and this sounds already simpler to me:

An app to install on Android / iOS just to be able to login (+ autologin), so they appear always online in the community when they have internet connection. Then only 1 function like a counter of new activities regarding their user account (new messages, new replies, etc..), and if they click on the app then a browser window will open to read the info at the main website.

So, what you think, it will be a big thing to develope such an app for the members? Is there a big diffrence between developing for Android and iOS? How to test the App if you don't have an Android or iOS phone in example?