Вот как это делает drupal .
В случае отказа сайта:
<?php
function pathauto_cleanstring($string)
{
$url = $string;
$url = preg_replace('~[^\\pL0-9_]+~u', '-', $url); // substitutes anything but letters, numbers and '_' with separator
$url = trim($url, "-");
$url = iconv("utf-8", "us-ascii//TRANSLIT", $url); // TRANSLIT does the whole job
$url = strtolower($url);
$url = preg_replace('~[^-a-z0-9_]+~', '', $url); // keep only letters, numbers, '_' and separator
return $url;
}