Where do I store a php file ( functions.php) in the laravel directory, so that I can call functions in it from any controller? I understand I am to include it in all the files where I have to use the functions, but I'm not very clear on where I need to put the file itself.
You can put inside in your App directory.
After that add the following line to your App\start\global.php file
require app_path().'/functions.php';
That's it.