Every time I open a new project or folder, I have to set the PHP version in the IDE along with the interpreter. I want to set 7.4 as the default. How can I achieve this?
Sure, just use File | New Projects Settings | Settings for New Projects...
and set the settings there.
This will be applied for every new project that will be created in the future on that computer.
Obviously, this has to be global PHP interpreter (that is available on the same OS where you run PhpStorm) and not project-specific one (e.g. Docker based interpreter is project-specific).