setup custom .env based on header token

I dont think you can change the .env. You should find another way of setting your db connection

You should be able to load an additional .env to override vars with Dotenv::load(DIR.'/../');

Behind the scenes, Laravel is using https://github.com/vlucas/phpdotenv which you might be able to leverage for your own needs. For example Dotenv::load(__DIR__); and also look at nested values if your sub .env file is going to have a persistent naming schema.