How can i make a Web Application?

Question

I already know HTML, JAVASCRIPT,CSS, and PHP
What should I learn to make a APP that works in Website for example : online photo editor or Video format converter.

Answer 1

You can make a web 'app' on the Code Playground. Ok, it's not an app, but it's the closest you'll get to making an app on Sololearn. As for the photo editor, you can take inspiration from this code:
https://code.sololearn.com/Wyb09U0Esl90/?ref=app
Just ask Brains before you do anything!