Prestashop and smarty php code

I want to include in prestashop (smarty) in the folder "/themes/my_template/modules/my_modul" a php file in the tpl.

example:

    <?php
include('/tools/smarty/Smarty.class.php');
$smarty = new Smarty;

$vorname="Horst";
$nachname="Meyer";

$smarty->assign('vorname',$vorname);
$smarty->assign('nachname',$nachname);

// ausgabe
$smarty->display('my_template.tpl');
?>

but the variable {$vorname} is not displayed. what am I doing wrong?

Copyright Notice：Content Author:「mirko」，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/27133447/prestashop-and-smarty-php-code