Question: I'm trying to create simple function that can be executed on **any page**.

```
function something() {
    $string = 'Hello World'; 
    return $string;
}

```

Let's say I'm in the category page, I would just call `$a = something();` and it would return my value

Platform : OpenCart

*P.S. I'm still studying MVC architecture*

Answer: 
Since you are wanting to understand and learn about the MVC system, the correct way to go about this would be to create your own helper file and put it in `/system/helper/` and then add the helper to `system/startup.php`. Take a look at how the `json.php` / `utf8.php` are done in these as a guide
Matricore's answer worked perfectly for me, but I also had to construct the new class with the config and db available.

Code below:

```
class yourclassname {

    public function __construct($registry) {
        $this->config = $registry->get('config');
        $this->db = $registry->get('db');
    }

}

```

You can then run queries via $this->db->query("Your query here");