When I was using PhpStorm I got best practice suggestions with yellow underline and a light bulb to for example use const instead of define. Example:
define("MENU_MAIN", "main-menu");
define("MENU_MOBILE", "mobile-menu");
define("MENU_FOOTER_1", "footer1");
Now I am using VSCode and can't find any similar functionality. Is there any extension or something?
There may be extensions that can highlight sections like this for code quality (I could not find any popular ones doing a quick search). However, there is not anything native in VSCode that does what you are looking for.
I'd suggest looking into a tool like PHP_CodeSniffer which doesn't care what IDE you use.