I am 31 years old and graduated university in math with a PhD in 2016. Since then I started a startup with my parents (my parents do networking, I do the website with Laravel/React for a 5k community, learning stuff on my own).
My goal is to transform into a PHP freelancer who works for a 3-9 months in different companies a year and the remaining months for own projects or the project for my parents.
I created a LinkedIn profile and got many request for full-time jobs which I declined until now because I wanted to keep the project with my parents going. I also got some 3 months freelancing offers, but they often included
experience with writing REST API's.
Although I have used many REST API's to include them in my projects, I never actually written a REST API so that other developer could integrate my REST API into their project (because there is no demand for this).
What would be the best way to learn this skill? Is that something that one learns only when working in a big tech or middleclass company?