OpenCart 2.x

The path names have changed in OpenCart 2 – you will want to create

admin/controller/extension/module/hello.php
admin/language/en-gb/extension/module/hello.php
admin/view/template/extension/module/hello.tpl

Then the route becomes

admin/index.php?route=extension/module/hello

OpenCart 1.x

• Include full MVC flow.

I found out how to do this. OpenCart uses the MVC pattern. I recommend reading about How to be an OpenCart Guru? post about learning how the system works – this Admin workflow should also suffice for customer end.

1) Create a new file in admin/controller/custom/helloworld.php

Your filename and controller name should be the same in desc order:

helloworld.php

<?

class ControllerCustomHelloWorld extends Controller{ 
    public function index(){
                // VARS
                $template="custom/hello.tpl"; // .tpl location and file
        $this->load->model('custom/hello');
        $this->template="".$template.'';
        $this->children = array(
            'common/header',
            'common/footer'
        );      
        $this->response->setOutput($this->render());
    }
}
?>

2) Create a new file in admin/view/template/custom/hello.tpl

Hello.tpl

<?php echo $header; ?>
<div id="content">
<h1>HelloWorld</h1>
<?php
echo 'I can also run PHP too!'; 
?>
</div> 
<?php echo $footer; ?>

3) Create a new file in admin/model/custom/hello.php

<?php
class ModelCustomHello extends Model {
    public function HellWorld() {
        $sql = "SELECT x FROM `" . DB_PREFIX . "y`)"; 
        $implode = array();
        $query = $this->db->query($sql);
        return $query->row['total'];    
    }       
}
?>

4) You then need to enable the plugin to avoid permission denied errors:

Opencart > Admin > Users > User Groups > Admin > Edit

Select and Enable the Access Permission.

To visit your page go to

www.yoursite.com/opencart/admin/index.php?route=custom/helloworld