Create a service and put your common functionality in it. For example, you can name it ArrayService and register it in the container as array.service. You can then access this service from controllers via
$this->get('array.service');
and from commands via
$this->getContainer()->get('array.service');
So, your code will look something like this:
$element = $this->get('array.service')->last($array); // or ->arrayLast($array)
If you need the same functionality across several projects, make a bundle with that service and add it to the deps file of each project. Then it will be installed when you run the bin/vendors install script.