public static function between(
        $latitude1,
        $longitude1,
        $latitude2,
        $longitude2,
        $decimals = 1,
        $unit = 'km'
    ) {
        // define calculation variables
        $theta = $longitude1 - $longitude2;
        $distance = (sin(deg2rad($latitude1)) * sin(deg2rad($latitude2)))
            + (cos(deg2rad($latitude1)) * cos(deg2rad($latitude2)) * cos(deg2rad($theta)))
        ;
        $distance = acos($distance);
        $distance = rad2deg($distance);
        $distance = $distance * 60 * 1.1515;

        // unit is km
        if ($unit == 'km') {
            // redefine distance
            $distance = $distance * 1.609344;
        }

        // return with one decimal
        return round($distance, $decimals);
    }