Error TPL PHP Prestashop

I am working on Prestashop and I would like to retrieve the data from a table I created in my database. I've been on it for 2 days and nothing works, I'll go crazy.

Here is the code I put in my ProductController.php file:

    private function GetAvailableAttributes($id_produit, $id_compte, $note)
    {      
    global $smarty;  
$sql = 'SELECT id_produit, id_compte, note
        FROM `'.'ps_product_avis';
    $ess=mysql_result($sql,0);
$smarty->assign('contact', $ess);
    }

Here is the code put in the product.tpl file:

<li>test :{$contact}</li>

Here's the mistake it gives:

Could one of you save me and explain where the problem comes from please?

I thank you in advance.

Copyright Notice：Content Author:「Yuto McTwinky」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/45014018/error-tpl-php-prestashop