I think the original OP, was referring to PHP environment variables, which is the preferred way for storing sensitive information like API keys, rather than being hard coded is stored on in a config file.
It would be nice if this is something that DirectAdmin could provide an interface for. It is common in local development to use a .env file, but in production you don't usually want a file with all keys are such readable, instead you would create PHP environment variables to then reference in PHP code instead, with something like getenv() or $_ENV.