Problem with foreach in Model

[eluser]Shpigford[/eluser]
I've got the following function in a model:

Code:

function getCategories()
    {
        
        $this->db->select('id, name, parent_id');
        $this->db->where('type', 'product');
        $this->db->where('active', 'Y');
        $this->db->where('parent_id', '0');
        
        $query = $this->db->get('categories');
        
        foreach ($query->result() as $category)
        {
            $output .= '<option value="'.$category->id.'">'.$category->name.'</option>';
        }
        
        return $output;
    }

But when I try to use the function I get: "Message: Undefined variable: output"