convert value from routes/uri to string

if you are willing to help me to change it to a query builder I am very happy.

PHP Code:

   public function user_campaign($key, $value)
    {
        return $this->db->query("
            SELECT *
            FROM (
                SELECT *
                FROM users
                WHERE $key = $value
            ) users_campaigns
            INNER JOIN campaigns
            ON users_campaigns.id = campaigns.id
        ")->row();
    } 


I don't see why you are using the sub-select. Here is how to do the above using Query Builder, but without the sub-select.

PHP Code:

   public function user_campaign($key, $value)
    {
        return $this->db
            ->where($key, $value)
            ->join('campaigns', 'users_campaigns.id = campaigns.id', 'inner')
            ->get('users')
            ->row();
    }