Don’t just stick to one or two technology! Explore stuff you don’t know that exists, increase technical breadth.
Learn how to enable HSTS Preload and tell the web browsers to load your site over HTTPS & HTTPS only, use SSL certificate properly!
Singleton Design Pattern is a part of one of the core concepts of Object Oriented Programming, Design Patterns — must needed for successful project.
Questions to practice / learn while you are reading on Memory Management to have a clear concept.