global variable in laravel

Sounds like a good candidate for a configuration file.

Create a new one, let's call it calculations.php:

Laravel ~4ish:

app
    config
        calculations.php

Laravel 5,6,7+:

config
    calculations.php

Then put stuff in the new config file:

<?php return [ 'some_key' => 42 ];

Then retrieve the config in your code somewhere (note the file name becomes a "namespace" of sorts for the config item):

echo Config::get('calculations.some_key'); // 42 in Laravel ~4
echo config('calculations.some_key'); // 42 in Laravel ~5,6,7+

Set a property on the BaseController, which should be located in your controllers directory.

Your controllers should extend the BaseController class and thus inherit its properties.