function makeSeo($text, $limit=75)
    {
      // replace non letter or digits by -
      $text = preg_replace('~[^\pLd]+~u', '-', $text);

      // trim
      $text = trim($text, '-');

      // lowercase
      $text = strtolower($text);

      // remove unwanted characters
      $text = preg_replace('~[^-w]+~', '', $text);

      if(strlen($text) > 70) {
       $text = substr($text, 0, 70);
      } 

      if (empty($text))
      {
        //return 'n-a';
        return time();
      }

      return $text;
    }

You can add more filters to clean the url and may be you add some more stuff to get that url a unique.
Note: I am not saying that adding url to the database is the best way. You could achieve the same sort of functionality using other techniques, for example, mod_rewrite.