How to create 500 error with the help of PHP

How to create error with PHP like 500 (Internal server error), 502 (bad gateway) and 521 (Web server is down)

I am not asking how to prevent or custom error page. I want to create issues/error to my server.

What code with PHP or Javascript i can do to bring my website/webserver down.?

Note : I am not attempting hacking to someone. I want to down my own web server temporary for testing and personal work.