SEO friendly links in PHP

This code removes all special characters from the given URL and make it SEO friendly.

   <?php        

    function makeMyUrlFriendly($url){
        $output = preg_replace("/\s+/" , "_" , trim($url));
        $output = preg_replace("/\W+/" , "" , $output);
        $output = preg_replace("/_/" , "-" , $output);
        return strtolower($output);
    }
    ?>