How to call function from custom helper ?

Sadly, I could not have found that solution in the documentation. It shall be explained more clearly. ( for example, I could not have found anything about the suffix name_helper.php).

So you can use the above solution or add your HELPER FUNCTION to the BaseController

example

/**
* An array of helpers to be loaded automatically upon
* class instantiation. These helpers will be available
* to all other controllers that extend BaseController.
*
* @var array
*/
protected $helpers = ["form", "test"];