Short answer: yes, it's possible
but, there are some concepts that you must understand before start.
- API(Application Programming Interface): are a set of methods that you will use for interact with your app. For example: get your posts in facebook, get the friend's list, publish a new post, update your profile, etc.
- Webservice: basically is a software component that let you share information between applications in a network (more at http://www.w3schools.com/webservices/ws_intro.asp). It's a kind of API that use http for communication. Are many options, but personally the most I use is REST with JSON. (https://stackoverflow.com/a/840713/1359796)
You must decide which information you want share between your web and the app. Maybe you just want expose new content to the app, in that case you will need just few 'get' methods. But maybe you want add information from the app to the web, in that case you will need 'set' methods, and also authentication (user, login, pass). Can be so complicate how you need it ;)
Then, go to work :) http://blog.ijasoneverett.com/2013/02/rest-api-a-simple-php-tutorial/ also can be http://www.androidhive.info/2014/01/how-to-create-rest-api-for-android-app-using-php-slim-and-mysql-day-23/
With that, you will have a website that 'talk' to others apps which can connect to it's API and can send, receive and update information from other sources.
Now you go to the other side: the mobile app. There you have hybrid or native solutions, but you want 'learn mobile app development', so go to native.
When you will understand the environment (and the lang), you can go directly for your solution:
note 1: many of those links I didn't read complete, you can use just like examples and I enforce you for find others by searching in the web.
note 2: Android and iOS are similar but complete different worlds. I think is better you understand the main concepts, but start to develop just in one of them. After you feel confortable, if you want continue with the second, great, but start with both at same moment can be a hard task.
note 3: if your side use some CMS, sure will exist some module (plugin) that can help you.