Laravel: How can I set an env variable using Config::set?

I'm trying to update env variables with Config::set and its working when I run it on my controller but when I move the code into a subfolder it doesn't work. The path i'm moving it to is app\sub_folder1\sub_folder2\MyFile.php

I tried creating a ConfigServiceProvider with the code below but not sure if that's even correct. How can I update the env variables?

 public function boot()
 {

    config([
    'app/sub_folder1/sub_folder2/MyFile.php',
    ]);
 }

This is the code that works on my controller.

 \Config::set('mail.driver', "smtp");
 \Config::set('mail.host', "smtp.gmail.com");
 \Config::set('mail.port', "465");
 \Config::set('mail.encryption', 'SSL');
 \Config::set('mail.username', "xxx@gmail.com");
 \Config::set('mail.password', 'xxxxxx');




 $from_email   = "from@gmail.com";
 $to_email     = "to@gmail.com";
 $subject      = "my subject";
 $body         = "body12345";

 \Mail::send('emails.test', ['body'=>$body],function ($message) use($from_email,$to_email,$subject) {
 $message->from($from_email);
 $message->to($to_email);

 $message->subject($subject);

 });

via Chebli Mohamed