pathauto_cleanstring

    /**
     * cleans up strings so they can be used in URLS
     * @author "Borek" - attributed to a post located at:
     *        http://drupal.org/node/63924
     * @param   string $string the string we're cleaning
     * @return  string  the input string, ready to go
     */
    function pathauto_cleanstring($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;
    }

For Drupal 8 and above, you can use the following code:

$clean_string = \Drupal::service('pathauto.alias_cleaner')->cleanString('Your very dirty string, with many URL un-friendly parts!');`