How to call a config item

[eluser]satie[/eluser]
Yes, sorry about lack of code.

@InsiteFX, thanks for your answer but if i put your code in my model CI returns

Severity: Warning

Message: Missing argument 1 for M_npanel::__construct(), called in D:\inetpub\vhosts\website.it\httpdocs\np_system\core\Loader.php on line 303 and defined

Filename: models/m_npanel.php

Line Number: 5

My question with more details
I have a model function

Code:

public function insert()
{
        $dati_annuncio = array(
            'password' => $this->funzioni->pre_password($this->input->post('password'))
        );  
        ...
}

So in my custom "funzioni" there is the 'pre_password' function:

Code:

function pre_password($password)
    {
         return sha1($password.$this->config->item('encryption_key'));
    }