Include this file in SMARTY_DIR/libs/plugins with this name : modifier.slugify.php

modifier.slugify.php

<?php
/**
 * Smarty plugin
 *
 * @package    Smarty
 * @subpackage PluginsModifier
 */

/**
 * Generate a slug
 * Type:     modifier<br>
 * Name:     slugify<br>
 * Purpose:  Sanitize a string for URL usage
 * {@internal {$string|slugify}}
 *
 * @param string  $string    string vers slug
 *
 * @return string slug
 * @author Grégory GERARD
 */
 function smarty_modifier_slugify($string) {
     $string = transliterator_transliterate("Any-Latin; NFD; [:Nonspacing Mark:] Remove; NFC; [:Punctuation:] Remove; Lower();", $string);
     $string = preg_replace('/[-\s]+/', '-', $string);
     return trim($string, '-');
 }