PHP Function: Search Engine Friendly URLs

Here is the function I have been using to make friendly url strings.
The second param is your spacer.

function makeURLFriendly($string,$spacer = "-") 
{
  $newstring = strtolower($string);
  $newstring = preg_replace('/[^a-zA-Zds]+/s', '', $newstring);
  $newstring = preg_replace('/ss+/', ' ', $newstring);
  $newstring = str_replace(' ', $spacer, $newstring );
  return $newstring;
}