Tips on creating a web & mobile application

I'm currently trying to learn new frameworks and languages, since it's summer and I have loads of free time. I've decided I should create an application that is deployed in both the web and mobile. As a developer who has knowledge in vanilla:

• PHP.
• JavaScript & jQuery.
• Android.

What frameworks should I use to build my app? Should I use a backend API for both apps, or should I seperate them?

Frameworks I'm considering are:

• For the web: MEAN Stack or Angular/React + PHP/J2EE.
• For mobile: React Native/Native Android + PHP/J2EE.
• For database: MongoDB or MySQL/PostgreSQL.

I'd like to know which frameworks should I learn first, and should I actually go with vanilla PHP for backend or a framework such as Laravel or Symfony.

P.S: I'm only trying to learn new stuff, I'm not looking to create a sophisticated app, but rather an app with an idea that requires learning several frameworks at once.